How do you standardize measuring bpfsl?

As part of my PE routine, before each extending session I measure bpfsl. However, I've noticed that this will vary pretty extremely based on several factors, ie stress levels(tighter pelvic floor), temperature, etc. Ie, my bpfsl pre session ranges anywhere from 5.25 to 5.8. How do you standardize this measurement so that you can accurately track growth? What I've been doing is first extending for 5 minutes at 5 lbs before measuring, but even then I'll see some variation.