There isn't just one factor for a given size tire, unfortunately. It depends on tread (how new the tire is), tire pressure, and tire load.

If you live where there are measured mile markers, you could put in the 20x1.75 factor and go ride a measured mile, noting beginning and ending odometer readings. You can then figure out how far you were off and put in the corrected value. It's not hard to get it right to within 0.5%.