Measuring

When measuring erect length do you slightly press down on pubic bone?

Yes press against your pubic bone but be consistent every time. If you press really hard 1 time and not so hard another you’ll get inconsistent measurements hence inconsistent results.

Yes this way you measure the BPEL (bone-pressed erection length). When you don’t push down on the pubic bone you measure the non bone-pressed erection length (NBPEL)

May want to stand while doing it, seems to give me mixed measurements also.

Stand, arch your cock forward, kegel, press the ruler in until you see blood, pray for that extra sixteenth of an inch. And do it consistently.