the best rpm to shift at is the HP peak......whatever that is for your engine.
I shift mine at 5800. but i have done quite a bit to my engine. they best way to find out where you hp peak is is a dyno of course, but you can figure it out by experimenting at the drag strip, by shifting at different rpms, say start at 4500, and then increase it by 100-200 rpm till you get the quickest time.
Keep you foot on the wood, otherwise you are causing the pressure in the transmission to drop, pressure is directly connected to throttle position, which could cause slippage when you get back into it.
2009 Harley-Davidson Road Glide
1996 F350 CCLB 7.3 5 speed 4x4