Some time ago Mike Hughes wrote about the alignment of the rear chrome strips on the fenders. However he also covered a cunning way to align the bumpers before doing the chrome strip alignment. Given that the bottom of the chrome strip should align with the bottom of the bumper it may help.
Quote from Mike
"The trim strip should be aligned horizontally, so that it appears from the side to be a continuation of one horizontal line, from the rear of the front wheel arch straight through to the rear bumper. The problem seems to be that when you view the car from a 3/4 front or rear view however (especially from substantially above!), the taper of the body tends to make it look like it is "off" somewhat. The descriptions given by the other responders seem to cover the proper alignment. Be aware that there is some play to the adjustment of the rear bumpers themselves, which are attached by only two bolts. You might try removing your rear strips, making a chalk line with a builder's chalk string, and then aligning the bumpers to the line before resetting your rear strips.”
Hope it all makes sense
Garry