Optimum shift rpm

Hi guys I am after some advice on what is the best rpm for me to be shifting on the 1/4 mile. I have a 1972 Australian Valiant Charger with a 10.2 to 1 cr mopar 528 cam 750 dp carb with eddy heads and rpm air gap manifold, 727 auto 2500 stall with 26 inch tyres and a 3.45 ratio diff. I currently shift at 6500 rpm and the limiter is set at 7000 rpm which it seems to easily rev to.
My question is should I be shifting sooner or later. The cam specs say 2800 to 6900 rpm range and the manifold says 6500. P.s I don’t have access to a dyno and only get to race a few times a year so I can’t just go to the track for testing.