I at least haven't made one, since I just use Calc.exe and use whatever I have available for me.

It's 20 points per skill level required (so 21 -> 22 is 21*20 = 420). 420 just happens to be 140*3, which means that three SR4s will do the trick without any waste.
Reminder:
Rs 10 points
R Grail3 60 points
SR2 70 points
SR4 140 points
SR Grail5 250 points
SSR2 99999999 points (I forget what it actually is, but it's enough for 29 -> 30 by itself).