I have this rule for some time. For AA50 i decided that the best option was:
Each factory you own take placement points and divide by 2, each factory you own this value goes up, each time a factory is bombed by SBR, every 2 points reduces this by 1. For me factories are also infrastructure ( roads, communications), so they must be able to be built up or reduced from the ravages of war.
Action: each SR point allows an additional movement in NCM of:
Double the speed within your starting contiguously connected land territories ( tanks move 4, inf move 2)
If you have an empty transport you can bridge up to that capacity a number of units across ONE sea zone onto another land zone. So if UK owns france and has 2 transports and 4 SR points cleared, they can transfer 4 more units to france from UK.
This only works with one intervening sea zone.
But then again this is basically AARHE stuff ( optional rules)