You need to watch a good tach at the finish line and use that for your calculations and use the calculator to figure the calculated MPH with your RPM, gears and tire sizes and then use your MPH to figure how much differences you have between the calculated MPH and your actual MPH.
Not all tach are as good as the better ones so choose quality over cheaper prices on buying your tach
BTW, I've seen and race with good race converters that slip over 10 % in the 1/4 mile, a really good one will slip around 2 to 5% at the finish line.