1/4 mile Shift Point

Download a g-meter app that works off the accelerometer chip, not GPS. Most phones that the screens turn have them.
Then, as mentioned, use a camera and the g-meter and tach to find where your acceleration drops, and start experiment shifting a few hundred below, up to g-drop.
This will tell you when the engine, in it's condition, wants to shift, not where anyone thinks it should.