What RPM should I shift at?

Easiest way for me is run the car up thru the rpm range under load - when it stops pulling you back in the seat - what’s the rpm? Remember that 5800 is for stock lifters and springs.

My 1972 Demon 340 4 speed 3.91 gear is shifted at 5800 even though It still pulls higher. 25.5” tall street tires.