Turn the low needle CW until it won't transition anymore. Then back it off CCW just until it will transition again (but a very fast transition). Your low needle will be perfect at this point.

Check your peak RPM. If it's holding a steady RPM for 5 seconds turn the high needle a little at a time CW and watch it gain RPM. Keep going CW until it won't gain anymore RPM. You should leave it the point of where it just starts to not gain any more RPM. Your high needle should be all set.

The high needle is furthest from the cylinder head. (Edited).

PS: As long as you don't run your engine wide open for more than 5 seconds or so you won't ruin it.