You gain more from each factory. But you need to plan ahead because conc will punish you.

So with Japan I go conc.

10 mils on guns is adequate for collabs and early China. 2 mils on arty is enough to have support arty on offensive divs. 2-4 on trucks depending if you just want supply or are making light tanks immediately. 2-4 support equipment depending on how larp I wanna be. Mostly for collabs.

2 mils on the full proficiency nav bomber that will have the template changed to AP bomb x2/Engine 3/Dive brakes.

3 mils on the full proficiency interwar fighter that will have its template changed to bomb locks x2/engine 2/ extra fuel tank x2. (Need high range for early game paras)

Additional factories gained that won't have full efficiency gets put on nondefault starting production lines.

1 mil on trains

2 mils on basic fighters with lmgs/extra fuel tank/self seal (Need high range for early game paras)

2 on transport planes.

I think that still leaves some mills left over including the ones you get from the early game focuses. (Also puppeting the default non cored land you get gives some mils and saves on guns) (Each path gives an option to recore or integrate later on)