When I had the calipers off my '84GL project car recently I noticed the difference in the angle between pistons on the same caliper. So, they do move out of position. The shoulder you are looking for is minute. At about 2/3 the way around the curve of the piston wall you'll see a slight bevel to the edge in two places which mark the "length" of the lower "step"

If you can remove the pads before the calipers, take note of there positions relative the front of the caliper. This way you'll remember approximately what they should look like when later you are holding them in your hand and things get disoriented altogether.

I put the nose of my channel lock pliers into the opening of each piston and spread the handle to get a grip on the inside od the piston in order to turn each one into orientation. I didn't measure anything, though, but Paul's protractor idea sounds good.