I worked for House of Fabrics during the big sales back in the 90s. I did it because I LOVED the 25% employee discount. They gave me NO training as to how to calculate anything for sewing so I just tried my best to hum a few bars and fake it. Women would come up asking me how much she needed for a wedding veil or how much for a sleeveless blouse. I would suggest they find a specific pattern. These stores don't give you any training at least in my day. They might have videos now a day.
You really only needed 15" so a half a yard is the right amount to get. I use to rip the binding along with the length of my border (2.5 yards) for my avatar quilt or any other Queen size quilt. Never had a problem. When you are working for someone on a custom quilt, you want to do it RIGHT and as quickly as possible.