In my opinion and this is sure fire way to figure this out. Take your car to a dyno, get a few pulls in, then check your torque, horse power curves and where they start and drop, that will help tell you where you need to shift. You can even data log shifts on a dyno so you can see what your power does in a down and up shift.
I usuall shift around 6500 rpms plus or minus for whatever I might be doing, my stock clutch likes to cook when I shift at red line. (swapping to a South Bend stage 3 setup very soon)
Here is a dyno sheet from Z1 Motorsports with uprev, and off this I would shift around 6800 to 6900 rpms but you should get your own results with your car, also view it with smoothing off might help out as well.
