The Golden Distance
There is a golden distance for models that have the Fleet ability. The idea is to take the most advantage of your extra movement to prevent the opposing army from shooting you. This has been made much more difficult with the advent of 4th edition rules and the option to move and still Rapid-Fire weapons. This obviously increases the threat zone of the basic squad and makes avoiding it more difficult. It is still possible, however. It just requires a good knowledge of the distance between your troops and theirs.
Without measuring, how do we know the distance, though? Math. Okay, you can stop screaming and holding your head, this won't hurt a bit. If you and your opponent set up your armies 24 inches apart and you move 6 inches straight forward, what distance now separates your lines? What if the opponent moves back 4"? Movement is measured, so you can know exactly how far models moved and add or subtract it from your original number. Most gaming tables are 4 feet wide. That's 48 inches, so if you use the "up to 12" in from the long table edges" type of deployment, you are, again, at 24 inches of separation. What other measurements can we use? I've already mentioned your own movement and your opponent's. What about any shooting from the previous turn? If that squad was in 24 inch range for him to shoot, how far away do you think he was? Obviously you can't use that if you have first turn, but the rest of the game will let you use that measurement. What about during deployment? Make a habit of watching your opponent's placments. Not to see where he puts his big guns, but how far he places them away from you. If he's about 2" back from the edge of his 12" deployment zone, that puts him 26 inches away from you. This concept works better with straight lines and small angles. It complicates things greatly if you try to assault one end of the enemy line with the opposite end of your line, running across the diagonal all the way. It also gives them longer to shoot you, so don't do it, okay?
So, now we've got the distance and the math covered, now what? Well, we need to find that sweet spot. We want to be able to charge them without taking a lot of fire. Rapid fire weapons can move and still double-fire up to 12 inches. Most infantry models can move 6 inches per turn. That gives them a large threat radius of 18" where their weapons are most effective. We want to stay outside this zone as much as possible, but still make our charge. The ideal range for us would be 19" for models with Fleet and a 12" charge, such as Hormogaunts or Leaping Warriors. The standard move of 6 plus a charge move of 12 gets us 18" and the Fleet will get us a minimum of 1" so we have 19 as our Golden distance. Even a little further away still gives us a good shot. The average Fleet roll is 3.5" so you can expect to go 21.5" on average. If you cut that to 20, you still get to charge on a 3+, which is pretty good odds. A very wise bug on Warpshadow.com came up with a shrewd solution. If you are 24" away from the enemy squad and you move up 6", you'll be at 18". Okay, so far there's nothing really earth-shattering here. Now, if you roll a 6 for Fleet movement, you'll be within charge range. Again, standard stuff. His suggestion was that if you get any result aside from the 6, you should instead move _backward_ one inch. That puts you at 19", the afore-mentioned Golden Distance. You won't get shot as badly and you've got a guaranteed charge next turn. Sweet! Your Hormogaunts and Leaping Warriors will dance up and down on the enemy squad and there will be much rejoicing!
Note that if you are using the "must deploy more than 24" from the enemy" type of deployment, you will be guaranteed to be outside of charge range on that first turn. Since you are placing units at this distance from the enemy, you can be sure that most of the enemy units will be less than 25" away. Good enough. On the first turn, don't even bother to Fleet. Just move up 6 and you'll be more than 18" away, which puts you at the Golden Distance.
Those models without 12" assault range have it harder, but it isn't necessarily all doom and gloom for them. There still is a Golden distance for non-leapers, it's just not immune to Rapid Fire. Rapid Fire only works at 12", meaning any victims of it have to be within 12" once the Bolters start shooting. Therefore, a brood that has say, 3 members within 12" and 9 members outside that range, can lose a maximum of 3 members to Rapid Fire. If the brood can fleet, the ones outside 12" can now reach the firing squad on their turn. (Since Fleet nets you a minimum of +1 inch.) Think of those first 3 victims as bait in your trap. There is, however, an outside range to consider. You have a maximum possible move of 18" which is also the exact range of a Rapid Fire weapon that moved (on an infantry model) It is unlikely you will get a 5 or 6 on your Fleet die next turn, so 17 and 18 inches are out. By the time you get to a 3-4, you've got a pretty good chance of making it, and if you've got another brood nearby, it's almost a sure thing.
From this, I think you want to stop about 15-16 inches away from the enemy unit. This is pretty close to a sure charge range, so they will have a hard time just sitting there and taking single shots. When they move up, they've only got a 2-3 inch overlap between the maximum range of their weapons and the lead members of your brood. If you've spaced the brood out a little, you shouldn't take too much damage, and then you are guaranteed the charge next turn. If they don't move up, they only get single shots, and you've still got a really good shot at getting the charge next turn. Less than 15 inches away and you are exposed to too much fire, more than 16 inches away and they may decide it's not worth it to move and you have to get lucky or be left hanging out in front of the guns for another turn.
Consider your formations when you space out those broods, however. If they are too tightly packed, the enemy will get a lot of kills with their Rapid-Firing weapons, since more will be in the kill zone. In addition, of course, enemy templates will also chew you up if you are tightly packed. On the other hand, if you are too loosely packed, not enough of your brood will get into combat, meaning you don't do as much damage and will take a bunch in return. The important number to remember here are 3 inches. The first three comes from the 15-16 inch Golden distance that we just calculated. The other three comes from the rule that allows all models within 2" of a model in base contact with the enemy to get their full attacks. (2 inches plus the width of a 1 inch base equals...) This means that we want to have a few models in the first 3 inches of the Brood. (The bait, if you will) and the rest of the squad in the next 3 inches at most. A 12-model Genestealer Brood might look like this from front to back:
__G__
G___G
__G__
GGGGG
G_G_G
This way, at 15" of separation means that the enemy squad will get 4 models at most from Rapid-Fire. Then with a 3+ for Fleet, the Genestealers will be able to get the remaining 8 members' full attacks! (Did I mention that Genestealers supposedly will get Fleet in the new Codex? :) )
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