Another tactic that I've used relates to that vertical line that Matt added to your drawings. If we pretend that the fellow is wearing p@nts, that would be where the side seams sit. So I measure my victims "half waist" and "half hips" -- that is, measuring from side seam to side seam around the back, then around the front of waist. Similarly I measure around the back of the hips. Adding the "front half waist" to the "back half hips" gives me my final hip measurement, plus a little for give/ease as Matt mentions.

The apron shaping question is a great one and I'll be following the discussion!