Today I did an unscientific experiment to check out where in the slow-down (regen) process the brake lights come on. I checked a number of data points at various speeds using a vehicle to follow me. Bottom line was that the brake lights tended to come on in a similar fashion as you would expect with an ICE vehicle with conventional braking. That is using regen to slow down was similar to the engine braking you would expect from a manual transmission to a point about half way through the first quadrant with no brake light activation.
More specifically, if you were to consider the regen gage starting at 0 degrees then increasing in a counter clockwise direction up to 90 degrees through the first quadrant the brake lights tended to come on after 45 degrees or roughly half way through the first quadrant on the gage.
For me in my Touring this was pretty much the case in all three drive modes and in both regen settings.
With hard lifting off the accelerator, the regen gage quickly goes past the first quadrant (90 degrees) and brake lights come on instantly.
Hope this gives a little insight without being too verbose and confusing. If so, kindly disregard!!!