Hi aaronr,
I was looking thru various threads today and I must have previously missed this one by a few months, I do apologize...
The answer is this:
In the 4L80E, the TIS sensor reads the reluctor teeth that are machined on the circumference of the forward clutch housing.
The forward clutch housing is driven by the overdrive gearset.
In gears 1, 2, 3, the overdrive gearset drives the forward clutch housing at 1:1 ratio.
In gear 4, the overdrive gearset overdrives the forward clutch housing at 0.75:1 ratio.
The TIS sensor reads the RPM's of the forward clutch housing.
If you pick a place your the log chart where the TCC is locked, you will see TCCSLIP is zero,
and here you can divide engine RPM by TOS and you will find it to be 0.75.