BPFSL - Measuring Issue

Did some searching but didn’t find anyone with this exact problem. How do you measure BPFSL? I am uncut, so I typically retract my foreskin, wrap my thumb and index around my glans, and pull out.

This OK grip leaves my thumb sticking out horizontally between my penis and the ruler, which I press into my pelvic bone and hold vertical. Then I have to look in a mirror and crank my neck to see where I’m at and it’s hard to be sure I’m lined up with the ruler properly.

If I crank my wrist so that my hand is underneath and the OK grip can be partially opened to where the ruler can make contact with my penis, I still have the issue of needing a mirror and still not being able to see quite where I am on the ruler’s marks, and also the stretch doesn’t feel as strong this way as I can’t grip as well.

Any tips on how to optimize BPFSL measurements? Ideally I would like to be able to measure accurately within 1/16th-1/32nd of an inch, and taking regular measurements are crucial for my routine.

Thanks