You and bearisgray are completely right that the more seams, the more fabric in the quilt. And Ginger is correct in pointing out that you have to consider fabric width and how you will be cutting the pieces when you calculate how much yardage to buy. However your calculation of using only half as much fabric for 8" chevrons as for 7" ones seems a bit extreme. I don't know what method you were using to calculate (whether fabric length needed, or fabric actually used in the quilt) so can't check your math. Can you show us how you came to that conclusion?