Easy fix. Look at an older pic when your bio hairline was “good”..
Use that as a reference point (use your temple points as a marker). That’s what I do. Could I go lower? Sure, but it won’t seem natural.
Or, I do the 4 finger measurement (4 fingers from top of my eye brow to hairline).works like a charm. However, I’ve always had a high hairline (even when younger). You may want to try 3 or 3 1/2 fingers Again, try to get the best balance between old pics and some kind of measurement system.
in terms of what’s the most accurate depiction of your hair - your phone’s front facing camera. Sometimes, zoom can distort the image. The mirror basically flips you.