As the crow flys...

This is not a big deal, but I have a slight upward arch in my penis when erect. When I measure for EBPL, the tape only touches the pubic bone and the head and stretches over the rest of the arched shaft ("as the crow flys").
When I lay the tape the whole length of my penis (into the arch), I get a little more than a 1/4 inch in length MORE than measuring the previous way. So, i'm just wondering what the accurate measurement would be?
Thanks for any help.

Probably try to straighten it out or lay the measure tape down along the curve.

Even if there is a curve it is still useable length therefore it should be measured. Measure your penis with a measuring tape this way the curvature is included.