Pete also posted data showing that his cal constants went up as his same tire wore away. My airless tire is showing the same thing.
I prefer wheel radius or "effective wheel radius" to "tire radius." It is a very complicated situation when you add in spoke and rim expansion and Oscar's own pet radius increaser, damp adhered particulate. Can't find Sanford article but have wondered if ride on the rough section where there might have been fresh tar was later in the morning . If stickier tar had picked up rock and increased the effective wheel radius, then that has to be a consideration. I have observed damp morning trails leaving a bunch of grit. Rides on hot afternoon sun trail sections leave little or none. Cal numbers match that theory. I think that is why Pete concluded you are better off calibrating on pavement to measure a trail surface.
As for intermediate readings, none of those are any good till you recalibrate. I do the same thing but the major use is when I mess up and have to back up the same day. Front flat and your day is over.
I gave up on 65 lb, high air volume, knobby tires after having to reride a section of a 5K course to get course > .0008. Same thing happened with 90 lb tire on fast heat up day when I rode 8 miles out, flipped bike and retraced 8 miles.
Must admit I do not understand what wheel friction has to do with "effective wheel radius." But, there is NO doubt in my mind that hotter, higher volume tires recalibrate to lower constant than either airless or higher pressure / lower volume tires. Only time I have seen an exception was on trails calibrated damp and recalibrated drier.