I find it very helpful to start a couple of airbase and port upgrades that I'm going to make anyway, so they give me a small amount of Practical boost shortly before the factories are done building. I also build enough "other" stuff to at least warm the Practicals for Infantry, Artillery, Armor, and Light Aircraft. Generally, I'll put one or two of those new factories at the bottom of the list, so they're getting partial or no production progress for a week or so, and rotate them so all but 2 end up delayed by varying amounts. The ideal is to have your airbases complete up a month before the IC, to offset the Practical decay and bring the completion date of the factories back on or ahead of schedule, have 2 factories complete roughly on schedule, and then the next pair of factories that are a week or so behind the first complete the following day due to Practical gain, with the rest coming up behind in rapid succession.
I've seen over a 2-month difference in completion times between the beginning of a run and the end....and I'm not even being extreme about it, that's with just 8-16 IC. With 20+ IC, it's a bigger change. A second run will give you even shorter build times for the last of those units.
The first 1-2 factories will provide a SIGNIFICANT increase in Construction Practical, so the rest of the run will finish several days or even weeks ahead of time. If you started them all at Jan.1, 1936 and kept them at the top of the que, then you just threw away those last weeks of production. I try to rotate all but around 2 of the factories to the "iffy" bottom of the que at some point, so their completion dates gradually fall on various dates about 2-6 weeks behind the first couple of factories. When the first pair of factories complete on schedule, the stragglers all seem to finish either the next day or only a few days later, thanks to the huge boost in Construction Practical. If you were putting full production into all of them during that entire 11 months, then those last 2 weeks worth of IC were totally wasted, because you could have used it toward other units and still completed the factories. You don't need to stagger the starting dates as long as you can divert production days to delay and stagger the ending dates.
Ultimately, as GER, the additional IC versus the cost of building it will not significantly affect Poland/France one way or the other, assuming two runs, since the time for it to pay itself off happens to occur in late '39 or beginning of '40. It WILL provide a significant IC advantage by the time you launch Barbarossa. As the SU or US, NOT building IC is silly, because you have the resources to fuel it, and you're not likely to be in the middle of a life-or-death fight until long after the additional IC has more than paid for itself, whereas building the military units sooner means that you have to pay to supply and upgrade them over and over during that entire time.
Let's see, tough choice:
...I can use my base IC to build 100 military units, and use that same IC to supply and upgrade them for up to 4 years until I need them.
...or I can use my base IC to build another 100 IC, and then use both the new and old IC to build the same 100 military units and about 20 more, all current, shortly before I need them in about 4 years.
