Even if you do a gearing calculator you're only calculating theoretical top speed. You aren't accounting for pushing against the air and weight.
I was talking to Brent about this because apparently he does know how to count. The speed for any given RPM and gear is independent of power / air resistance, assuming you have the power to get there. So if you are on the limiter in 6th, the calculator should be correct.
I compared my GPS data (I use the R1 YEC data system in the R6) using multiple different gearing ratios to the calculator. I also compared data from my stock engine to my rebuilt more powerful engine. The calculator matches my data.
Make sure you enter the correct tire size info since Dunlop slicks are not stock size.