I'm going to start an afghan for a friend who just bought a new house. The measurements are 48x60 and as a guy who is 6' tall, 60 inches just isn't long enough for an afghan. I want to make it 72 inches instead. I'm wondering if a good way to guess how much yarn I would need for the extra inches would be to take the total yardage needed (2795 yards) and divide it by 60 inches to get 46.5 yards an inch. Then multiply 46.5 by 72 to get a guess of how much I would need.
Would that work? Is there a better way to do it?
I'm thinking about the Shivaree Throw pattern with a machine washable yarn.