What RPM should I shift at?

I just simply play the shifting games at the track and let the car decide. IF you are at 5800, then make the next pass trying to shift at 5500. IF the car goes faster or slower, adjust accordingly. I always try to shift at the least rpm that runs the best ET/MPH. Example: I shifted a car at 6k, but it ran the same at 5700, so I shifted at 5700 and it still was the best ET/MPH.