I still dont get it. Why have the monthly MP-gain, again?
Say, in germany (1937 borders) there are 10,000,000 avaiable men (just giving a number), and that 1,000 equal roughly 1 MP (yeah, ´quality´ counts, too, but that is reflected by the units´ MP-requirements). So, that would mean, germany has 10,000 base-MP. Now, before Anschluss, or whatnot, they can only utlilizte 10% of it. The various events dismantling versailles, like remilitarization of the rhineland and the Anschluss of austria raise it by 5% each. Austrua obviously adds the austrain provinces to germany, as well, raising the base to say 12,000,000, out of which germany then can draw 20% (10% + 2*5%).
Ideally, there would be a minimum of base-MP, defined by IC*tech_factor, going below which puts a malus on your industrial productivity.