ArmorMaster:
Since this appears to be "groundy", then I think you could set unit limits as follows:
-- Allied: Landed in Normandy between dates given, whether in combat or not.
-- Axis: Using Allied forward lines on any particular date, any Axis unit in lines facing the Allied units, or behind Axis lines but in direct or general support of front-line units.
I would limit air & naval to direct/general support within the sea lanes from the UK to the beaches and in the airspace over sea lanes & beaches and the air over the land areas as defined above for ground units. This may be too limiting for things like rear-area interdiction by air units & exceptions might have to be made if builder can provide documentation/rationale, but should exclude something like a B-17 on its way to Berlin.
Right now, I'm a tentative but haven't quite decided what to do -- it will be something US or UK either on the beach or in the hedgerows ....