1/4 mile Shift Point

You need to experiment with several rpm points and compare time slips. You are looking for shift points where the motor rpm doesn't drop to a point of being out of the power band. Basically if the motor is laboring and not pulling hard when you upshift you shifted to early.
A video camera recording of the tach lets you replay each pass and you can look at the rpm drop on each shift.
My 340 low gear set 904/3.91 runs best with a 6k 1-2 shift and 6200 2-3 shift.