I left it at 8500 rpm ,i  used the 87 zilla gear ratios from my manual
Its the calculator on everything two stroke, you have to be logged
In for it to work.
I tried different tire sizes also just to see, it seems you  could get almost 5 mph more for every
Inch taller in tire size .
Everyone knows its not going to be dead on all the time, but i think that
Calculator gives you a ball park as to  what to expect when you want more
Bottom or top end speed

His engine will not make good power at 8500 RPM.  His engine should make its peak power in the 7400 to 7800 RPM range at those speeds.  Maximum top speed will be obtained when the gearing allows the engine to run at its power peak.

Aerodynamics, rolling resistance and the amount of peak power is what determines the maximum top speed any vehicle can obtain after the gearing has been optimized.  The rider stance / position, front and rear fender cut is also a big part of the aerodynamics of bikes and quads.

Most gearing calculators do no account for tire growth.

Continue to take teeth off of the axle or add teeth to the counter shaft until it slows down.