Practicals don't solve the problem. The first set of factories have little or no practicals. These are your first ones so how can they? So for about a year you lose 5 IC. That is a fact. And a factoryu adds 1 IC per year so the math is it takes 5 full years to get back to even. Since that starts in 1937 that means it is 1942 before those first factories pay for themselvs. Where am I wrong here?
War economy doesn't hit till 1939 so the best that these might do is breakeven in 1941. Here is the math as I see it. Let's use the following examples. In 1936 you build 10 factories for a cost of 50 IC for 1 year. In 1937 you build 10 factories for a cost of 45 IC for 11 months and in 1938 it is 40 IC for 10 months. BTW your July 1938 is impossible as your first build doesn't benefit from the 10 month practical. The numbers I'm using are tilted in your behalf and probably offer larger practicals then you can actually get.
Anyway you lost a grand total of 45440 IC (50 *365 days + 45 * 334 days + 40 * 304 days). So lets look at what you get back extra. For a war starting in September 1939 that means you get only 20090 IC (2 years 9 months * 10 + 1 year 10 months * 10 + 11 months * 10). So for the 1939 start your army, navy and airforce had a grand total of 25350 IC less to build stuff with.
Now lets look at June 1941 and see how it goes for that date. You get a grand total of 39170 IC. The 3 months in 1939 plus 1940 plus 6 months in 1941 times 30 IC nets an additional 19080. That is still 6270 IC short of not building any factories.
So unless my math is wrong it shows that building factories will hurt your 1941 and especially your 1939 forces. By the end of 1941 the strategy pays off but I already have the bitter peace so not sure what I need the IC for anyway.
Wrong. And you are calling me a liar by saying that
"BTW your July 1938 is impossible as your first build doesn't benefit from the 10 month practical."
I routinely do this and end up getting 3 cycles of 14*3 done before or on July 1 1938. Since your premise is wrong, your math is irrelevant. As to payoff, I don't really care about that as any comparison with that would also have to include supplies + upgrades for units produced instead of factories built. My factories finish by that date, or even earlier due to researching ahead of time the 2 IC techs. And you
never once mentioned modified IC which is 55% at normal difficulty. So your response is completely irrelevant.
Actual math for 14*3 factories for Germany in vanilla HOI3 where Germany's construction practical is 5 [with CPO]:
4.75 IC for 351 days = 1667.25 IC*days
with modified IC at normal difficulty, you get a 1.5X payback =
1111.5 days to get payback [1667.25/1.5]
which is a little over 3 years for the first batch. Then you get better payback periods for the next 2 cycles getting to around 2 years for the 3rd cycle.
Factory build time will finish even quicker with Mixed Industry or by researching ahead of time the 2 industrial techs. The 1938 techs will finish along with the first 14 factories, while the 1940 techs finish around Nov/Dec 1937 IIRC. I do this all the time and track it with screenshots and spreadsheets so I know what I am talking about. As to whether or not it's a useful strategy, players can decide that for themselves once they have the facts, and I have given them the facts for this setup [vanilla HOI3 1.4 normal difficulty].