how to get true vertical 3" a-pillar chop on 54 chevy?

hi, i have been trying to figure out how to get a true 3" verticle chop on my a-pillar of my 54 chevy 2 door post. i will be chopping the car 3" in the front and 4" from the b-pillar. my friend and i have been through a million different scenarios to try to get an exact measurement before i cut it. it seems that if i wrap a 4" piece of tape around the a-pillar, it should come out to be a 3" verticle chop. i just thought i would ask some advice from others who have done this before i cut mine. any info will help. thanks

 

make a mark on the pillar where you want the top of the cut.

hold a plumb that is 3" long (3" total including the bob and string) from the edge of that mark. use a level (the longer, the better) to make your bottom mark. simple.