Atension I have to say I think you're over emphasising the effect of countercharge quite a bit. In my experience what you describe in theory is actually fairly rare to pull off in practice. Mainly because you have to countercharge towards the nearest enemy, not the assaulting formation. Especially with a larger formation, occasions where all of the assaulted units are able to move in a single direction entirely out of support range would seem quite rare (to me).
The assaulter has to choose how to position the units. If he wants support, his supporting units need to be within 15cm of the same units that are within 15cm of the assaulting formation, which you have a few options for:
1. If you assault from the same direction (i.e. adjacent to your support), then countercharges will be moving towards support, not away from it.
2. If you assault from the opposite side of the formation then, to get within 15 of the directly engaged units, your supporting units will be closer to the "back" of the target formation than your assaulting formation is. Certainly that is the case if the target is a large-ish formation or is spread out (due to potential barrages). Remember you only need the support formation to be in range and closer to a single unit in the target formation to prevent them counter-charging away:
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The only way I see a large formation of jetbikes being able to get completely out of countercharge range is if the attacker deliberately moves their assaulting units up very close or in base contact AND has left their support quite far away. That will likely place the enemy's back units closer to your engaging formation than your support.
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But against wind riders there is literally no advantage to doing that because you can't CC them anyway. Just keep them as far away as you can - if the jetbikes are countercharging towards you, it means you didn't have any support units close enough in the first place and the 10cm countercharge is not a factor.
What 10cm countercharging jetbikes
are good at however, is not being clipped. They generally have the move to be able to get all their bikes to fight. But again, placement of support (closer to the formation than the assaulting one) can prevent that too:
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