By France 1 there is generally:
1 Cruiser
2 Destroyers
5 Infantry
However, there is 8 IPC in land value pretty much unclaimed.
The idea of the rule is to give England a boost since they are dumping more than they earn into defending London from Sea Lion. (well, they need more than they earn if Germany has decent dice and sets up an early Sea Lion.)
However, perhaps you want to alter the numbers a bit. /shrug.
1-2 its British unit(S), territory(ies)
3-5 its French unit(s), territory(ies)
6 the units are removed and the territory is left empty.
Gives England a 33% chance to pick up any remaining French Units and Territories.
Also, if you add in the rule the French units cannot move for any reason after Paris, France falls, it effectively ends France from needing a turn and tying up the game. Italy can go, then Germany in one smooth, uninterrupted turn. And no more 3 day waiting periods for France to say “do nothing, go Germany” in online play!