Don't worry about the size for this project. I did a card, and being really concerned about paper waste (plus I didn't have seven different shades of the same color), I just measured from the center line (of the card) end to the
shortest point of the card edge. That way I only had to trim small triangles off the card after affixing the strips.
I hate
almost having the correct size paper for a project only to be unable to use it because it was 1/8" (or 3/8") too short on my leftovers and disliking having to use a full sheet of paper just for a tiny amount.
BTW, the triangles could be used in another paper quilt design.