Hi skibum,
That's an excellent question. It has been my experience that a one letter change in the needle straight is enough to split the difference between two different pilot jet sizes.
For example: Let's say you were running a S needle straight at high altitude (8500') and were withing range on the ps, 1 & 1/4 turns out to 2 &1/2 turns out, with a 40 pilot, and when you came down in altitude to say 3000', you find that you are out of range on the ps, more than 2 1/2 turns out on the ps.
So you switch to a 42 pilot but now find that you are less than 1 1/4 turns out on ps. There is a good chance that just switching to an R needle straight and that will set you right with the original 40 pilot will set you right.
This should make sense as the difference between a S needle straight @ 2.765 mm's and the R needle straight @ 2.755mm's is .01mm's. And the difference between a 42 pilot @ .42mm's and a 40 pilot @ .40mm's is .02mm's. So changing the needle straight on paper anyway splits the difference between the 40 and 42 pilot.
You'll just have to make some experiments like I listed above to make sure it's right.
Hope this helps,
Dale
