https://www.sigmasport.com/en/allgeme...C_1200_nat.pdf

I'm running 26x1.95 tires.

26x1.9 - 1298
26x2.0 - 1313

so 26x1.95 should be 1305.5? Right? 1200 doesn't do fractions/decimal, so is it better to round up, or down?

Unless you keep your tires digitally inflated at a constant pressure within a tenth, it doesn't matter.

You would think, but I was running 26x1.5s and the same one way run came up as anywhere from 3.53 to 3.55 miles.

Using 1306, same run came out as 3.68 miles this morning.

The only truly accurate way is to do a rollout. Put something on the tire of the wheel with the sensor that will transfer a mark to the ground. Then ride the bike with the tires inflated to your preferred inflation pressure. Measure the distance between two marks, and that is your number (in mm).