Rover owners: Measure twice and drill once

I have a set of Parking sensors to fit to my '97 MGF.

The bumper is smoothly curved all over, but the bulges change over the length of the bumper, so I am not sure how to measure up to get them level and even.

I was thinking of measuring from the floor to points on the bumper but I am not sure the car or floor would be level. Scratch that idea.

If I measure from the bumper seam the inner ones should be the same and the outer ones will have a different but matching measurement. how do I correctly line the two sets up.

Could I use an electrical trunking lid to provide a straight line?

there must be a tried and tested method for this but I just don't know it.

Here is a bumper pic (not mine)