I take a piece of string, tie a small loop in the end and then basically "lasso" the tire.
Mark the string where it overlaps and then hook the loop on a nail or something and stretch the string (approx same tension as when in "lasso mode") and measure it with the tape. Repeat until you feel you have a consistent measurement.
Divide that # by .03937.
As mentioned, a difference of 20+ out 2000+ is pretty small.
Or you could just enter 2111.