We've discussed this before and I'll try to find some more extensive explanation to link for you. But basically, you need to measure your hips to include the "bay window" or jutting out portion as you call it. The front apron should hang straight down from the waist to hem. I do this by:

Imagine you are wearing p@nts that have side seams. Measure the front half of your waist from side seam to side seam. Then loosely measure the rear half of your. . . rear. . . from side seam to side seam. Add those two numbers, and make that your hip measurement.

Make sense? As I said, there's more discussion of this in older threads, I'll see if I can come up with some for you.