Mark, you are correct. You need to measure the roll-out of the tire with the pressure you are running it at. The advertised diameter x Pi vs the roll-out may contribute to a small math error. That said, the Goodyear 2286 (28.0x4.5x15) that we run rolls out at 28.01 at recommended (70#) pressure. Ideally, the driver would jump out at the end of his run and check the rear (usually) tire pressures so we could account for tire growth contributions to the calculated vs. actual measured speed discrepancy. Since he's just hanging out waiting for his recovery vehicle, he could carry a pyrometer,too, and record the temperatures.
From what little technical data I have seen, the above tire may have grown between 0.50" and 0.75" at 250 mph (where we run), so that growth might account for a small difference. If anyone reading this has real data on 2286 growth, I'd love to know it. On what I consider a "good" run (no obvious traction problems, no shifts that are too slow, etc), the exit speed on the time slip is damn close to the math calculation. Usually less than 1 mph, probably more commonly .5 mph.
The other thing that contributes to small calculation errors are the rounding up/down of numbers. Many people consider 4:11 to be the accepted r&p ratio, but the r&p in our quickchange is much closer to 4:10. You need to be aware if/how your spreadsheet rounds up/down your gear ratios, too, if you want to account for other small math 'error' inducers. As I know you are aware, all the little things add up.
I look forward to seeing the Midget set the record (last drove my F/P Midget @ Blackhawk in '79, I think) and to renewing our 40 year old acquaintance at Bonneville.
vic
PS: Stan, you are correct, too.