@barnee re: testing
Since the Axis each have a starting income that is divisible by 3, originally I had the mechanics for merging theatres as a function of the Axis country either increasing their income by 2/3rds more than their starting income, or decreasing it by 1/3rd less than their starting income. (Which sort of dovetails into the “2 out of 3” rule for Victory Cities.)
This effectively worked out to:
Germany: +14/-7
Italy: +8/-4
Japan: +10/-5
Changing it to a uniform +10/-5 made the math a little bit more intuitive, but it also means the USSR doesn’t have to get beat up so much (down half their income, possibly even losing their capitol) before the US/UK are able to jump in and help.
In terms of territorial gains/losses, this typically works out like…
+10 Germany = Karelia, Ukraine, Caucasus, Persia
-5 Germany = Finland/Norway, Eastern Europe
+10 Italy = all of Africa, plus Brazil or Eastern Canada
-5 Italy = all of Africa (except for one territory), or losing Southern Europe
+10 Japan = all Chinese territories, plus India or Philippines
-5 Japan = Indochina/Burma, Manchuria
So, Italy probably has the toughest time (as one might expect) but none of these scenarios are completely out of the realm of possibility.