yeah feel you in general there. This is one of those thing where just stating it explicitly gets you in a better place.
The thing about simple is good until you become inconsistent. I mean distance DOES mean something in most of the important rules already (Shooting, Coherency, Supporting Fire, and Close Assault all being prime examples).
Personally I'd go for the 5cm thing as it's simple, already used in the free optional debus when assaulted, and already the most common style of rule we see in this level of game. The whole way EA handles transports is crap IMHO making them NOT actually work like the way transports work in Mech warfare.
I'll also point out that it has synergy with those bits I posted today on BaseCamp for discussion!

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