I put a piece of tape on my tire (just to get a marker closer to ground level), lined up a metal tape measure, then sat on the bike, slowly rolling it next to the take until the marker hit the bottom again. The way I did this was to use a hallway so I could hold upright by using my hands on the wall. Did numerous tests and they were the same each time so it was consistent.

I have also found that my normal ride is amazingly consistent - I hit 10 miles at almost exactly the same point every time - I expected more variation because of minor alterations in the exact line I followed. I guess the pluses and minuses evened out, so now when I put on a new tire, I see where it shows 10 miles and if not the same, I adjust it until it comes out right.

I know that weight on the bike makes a difference because I checked both ways. I don't know if minor changes matter much - at least not on a high pressure road bike tire. My weight has covered a 10 lbs range and I can't see any noticeable difference on my ride distance - lower pressure tires may vary more from less weight, I guess.

But if you do let the pressure drop, that will make a difference.