With flared bars becoming more common, even on the road, I don’t know how to avoid an inward pointing lever.

Is parallel with the lowest straight line of a handlebar adequate?

As for OCD. Occasionally marked bars are marked wrong. You really need to measure, both the middle of the bar and a spot somewhere on the curve to mark yourself, before the handlebar is even on your bike.

Theres a zillion different ways to measure a bar, probably a flexible sewing type measuring tape is the easiest.