I see now a way to make the count:
You put on the board a number of flag tokens equal to IPCs value when you win new territory.
And you give the original owner’s as many Flag token (FTk) from the conquerer than the face value of the territory.
When this territory is conquered back, you return the FTk in to the enemy’s hand.
When it is your turn, count every flag tokens (own and enemy) in hands, each is worth .5 IPC that you must pay before making purchase (if their is one FTk left, keep it for the next turn). So even a 1 IPC territory can be counted without approximation and rounding up or down. Just keep the lone FTk until you have to pay a whole number of IPC.
For example:
Russia take WRus 2 IPCs
It put 2 Russian FTk on board, give 2RusFTk to Germany= collect: 26 IPCs.
On Germany’s turn 41 IPCs minus (2RusFTk X.5 IPC) = 40 IPCs
Germany: win back WRus 2 IPCs, return 2 RusFTk to Russia= collect 41 IPCs
On Russia turn’s, 26 IPCs minus (2RusFTk X.5 IPC) = 25 IPCs for purchase.
So the initial global gain 4 IPCs for a 2 IPCs territory but we substract 1 IPCs from Germany for loosing the territory. And Russia the same.
So the real IPCs for purchase is equal to the value of territory on the board.
Although, we need to introduce a special rule for the first turn of play.
You don’t receive Flag Token for territories lost on the first turn of play unless you had already collected incomes.
Thus, Russia don’t give FTk to German player.
Germany give FTk to Russia but neither UK or USA.
Uk give FTk to German’s player but not to Japan.
Japan give FTk to Russia & UK but not to USA.
USA give FTK to Axis’s player. So every one give and take.
I think it is playable and not too complicated to use.
Many players already put nation’s markers on the board to keep track of conquered territories.
You just have to place the same number than the value of the territory and give as many markers to the enemy player (you’ll enjoy it).
I’m pretty sure many player’s will like to give Flag token to others… and won’t forget to check them and make them pay.
Now, the “more you invade me, the more I produce” paradoxe is fixed.
But there still have the other part of the problem to check: does it favor the balance toward the Allies as said Gargantua?
Are we able to keep the initial balance and movement of the game: Axis growing fast or get beaten?