Ok so I can now confirm that for my specific application, (2015.5 GM L86 Engine with 8L90e 8 Speed transmission) I was able to successfully use the "Accelerator Pedal Position" value that spans from 0% to 99.8% in lieu of the inaccurate "Throttle Position SAE" (that only spans from 15% to 83.9% even at WOT) value to work on the Part Throttle Shift tables.

The transmission uses the tables as a guide and is, in most cases, within 1 to 2 miles per hour from the tabled values.