As for the specific numbers I chose them for a couple reasons, first because I think that the real advantage of the Japanese tank drive to Moscow is more positional than income oriented. When Japan stomps Russia it’s not so much the 5 spaces worth 1 ipc that they’re after, but rather a way to get their forward attack units closer to the center. Under this rule Japan will have less incentive to drive across the northern route, and more incentive to push across China or the South, where it’s easier for them to converge their stacks. This means when they come rolling into Kazakh, Novos or Evenki they’ll likely be moving with a pretty large stack. By the time Japan breaks the NAP, the 7 inf or 21 ipcs the Russians will gain, equates to almost a full round’s worth of income, which I think is what would be required to prevent a simple steam roll.
In the KJF situation, the 7 inf/ 21 ipcs is really just a way to keep the Russian tanks from blitzing all over coastal china, after USA/UK can opens the area. What I’m really trying to avoid is the situation you see in Global, where Russia has no incentive to maintain the NAP, and just declares war against Japan in the first round, because there is no serious consequence for doing so. Here Russia would definitely stand to gain more by waiting, than attacking in the first or second round.
I went with 7 inf because, if Russia stacks bury with the 5 Far East infantry, that gives them a total of a dozen inf to face down a attack on J1/J2 (should they choose to rally the troops in Bury.) So they are able to stack defensively, but cannot really advance into Manchuria. It basically stalemates that northern border for a time. Alternatively, Russia could send these units into China (Mao style), where they can confront Japan indirectly on defense, instead of directly. I think this will make the NAP feel a bit more in tune with history, as opposed to an immediate all out conflict.
21 ipcs seemed to offer a few interesting options at purchase as well, as there are some nice combinations of units you could purchase for that amount.
Again this stuff is more abstract than one might prefer, but the trade-off and benefit is that its relatively simple to implement. You don’t have to explain things in terms of Mongolian units or whatever, its really just a gameplay driver meant to keep Japan off Russia’s back. Even in the KJF, the NAP still favors Russia over Japan (since Japan usually ends up adopting a strategy of stealing Russian cash or trying to operate up north as they lose ground in the south, and here that is clearly less advantageous.) So Russia has a strong incentive to just hold the line, and it’s Japan who stands to loose the most if the two nations go to war. Which fits I think with how most people see the history playing out. If Japan wants to say screw it, and crush the center, this means at least a 1 round set up to get into position, instead of just blowing the door open immediately, and then Russia gets a nice boost to help them deal with all the chaos that a center crush from Axis entails.
If the NAP is preserved, the income Russia will save each round, (since they don’t have to deal with a Japanese invasion from the rear) can be pretty significant. On R2 they will have 1-2 ipcs more than they usually do, on R3 they’ll have 2-4 ipcs more than they usually do, on R4 they may have 5-6 ipcs more than they usually do etc, which adds up over time.