I would be hesitant to estimate yardage, especially providing a swag estimate for a potential client. They won't remember much except if the actual yardage is more than the guestimate, they will remember the lower number you told them!
I have made full quilts that needed 13 yards and some that needed 6. That's a big difference. In addition to the number of seams, some folks are assuming a 4" drop all the way around, and some are thinking 14", so even the size is a moving target.