Starting Shift Point

Want to really find out?

Strap it to a chassis dyno.

The other way is to take it to the strip.

The way most simps do it is keep their foot in it until you feel the pull subside then shift. Not a very good method.
You can also do this with one of the in car performance monitors. What you want to do is generate curves where they intersect for the gear changes for "anything"---torque, HP, acceleration. You shift where the curve for 1st gear intersects the upcoming curve for 2nd gear.

If you cannot generate a curve for every gear, then generate what you can and "figure" the rest from the transmission ratios

best-gear-change-rpm-change-at-the-torque-intersections.png