I don't "do the math". But I have a general idea of where the revs will wind up. Basically, I'm doing it by "gut feel". I know that revs will be lower in the next gear higher. So I shoot for having the revs somewhat lower in the next higher gear. If I was going 4000 RPM before the upshift, I'll shoot for 3000 RPM after the upshift, and it will be approximately correct.
If you want to do the math to calculate the revs, check out this table showing "RPM Drop During Upshifts", from the Gear Ratios section of the FAQ. It shows the RPM after upshifting from redline (8000 RPM) for each of the upshifts, with the stock 5-speed, the stock 6-speed, and the short gear 5-speed:
1 -> 2 4499 5106 5085
2 -> 3 5698 5837 5738
3 -> 4 6289 6266 5903
4 -> 5 6378 6500 5971
5 -> 6 n/a 6330
You can do the math - so, for example, if I am upshifting at 4000 RPM, the revs after the upshift will be half (4000/8000 = 1/2) of the number shown in this table.
Again, I don't actually do the calculation; I just have a general sense of where the revs will be, knowing that they will be roughly 25 percent lower after an upshift (except a first-to-second upshift, which will be roughly 37 percent lower).
The revs don't have to be EXACT. It's better to try to match revs and be off by 100-200 RPM, than not to try at all.
