I checked BA's work(not that I don't trust him) by deriving the equation and constant from first principles. The math doesn't lie. You can also check against Steve Timmons gear charts which agree with the formula.
Gear charts
The tires do grow a small amount, but that won't change the result much. I don't know how to check the accuracy of the tach, I've always just sort of believed what I see in that regard.
You going to the Glen for the club races? I'd love to see your car.