Art De La Guerre

[madaxeman]

[vexillia]

[DieselDave]

Not sure how Herve's answer helps.
You still don't know where the c-in-c is so don't know if everything is in command range. You could ensure all the ambush markers are close enough together that the furthest a unit could be deployed from any other furthest point in another ambush marker is within command range.Of course you could do that with units that are deployed at the beginning of the game.
It seems to me that the simplest answer is to assume the c-in-c is free to sneak about within the ambush(which he is because you can place him anywere as part of the ambushing group when you deploy them on table) and therefore measure the range from the ambush marker.
Dave

[Andy Fyfe]

[Ramses II]

@DieselDave
There is no requirement for an ambush marker to be under the control of a commander, and when the ambush is revealed the units may be placed out of control - in which case they require +1 UD to activate. 

Also you mention the “c-in-c”; I presume you mean the commander, as the CinC may not be placed in the ambush. 

[DieselDave]

Oops!, yes I meant commander.