Another gotcha is if you swap to a different wheelset that uses different pressure and tire size. In practice, I don't think it makes a big difference but the purest in you suggests you want to calibrate it just right and that's ok.
I used to calibrate mine at normal pressure but without me on the bike. Once I tried the proper calibration of pressure any me on it, there was about an 8% variance. Mind you, once I switched from being a roadie to mtb I dropped the pooter and use my watch instead and time my outings if I need to.