I measure to the 16th of an inch. I also measure very often (sometimes daily) and keep a sort of running average in my head, to try to account for variance in erection quality.
Another thing I do is to measure every possible way. BPEL (sitting), BPEL (standing), NBPEL (sitting and standing), flaccid, flaccid stretched, erect mid-shaft girth, base birth, post-workout size (for different kinds of workouts), and I'm sure there are some I've missed.
The reason is because there is not a perfect correlation between those measurements. For example, my "standing" and "sitting" length do not change at exactly the same rate at exactly the same time. The biggest factor is probably a lower exit point as my ligs get stretched. But I've seen one measurement get bigger and the other remain completely unchanged.
This may not work for everyone, but it works great for me. Just measure consistently and you're all good.
Edit: I forgot to add that I also cut the end of my ruler so it starts at 0.