from my own testing of a variable openning point.... it seems the later toyota ECU seem to be half way correct in picking the right openning point for the tvis.
it should not be rpm alone based. you can do a dyno run with it closed and open, and at wide open throttle you have the best change point. but under less throttle and less boost its at much higher rpm points.
so basically its airflow that dictates the best point to open. at higher boost, you flow more air, and its lower. i guess the next best thing to try will be the AFM signal.
its clearly half the runners in the inlet plenum closed, will always increase air speed , and thus give better power, until at a certain airflow, you have enough volume of air that it becomes too high, and then all runners open, give enough air speed and the advantage of more easy to flow path thus making more power.