I read everything I could on this subject and found three so called correct stripe measurements. But I also read they did not come with stripes from the factory. The dealers painted them on. ERA had two measurements on their website. One with a 3/4" gap in the middle, the other with 1". For tapered stripes they seem to be wider by 1 1/4"-1 1/2" at the cowel edge of the dash than the nose. The rear deck is the same as the dash and then tapers a little less 1 1/4" below the trunk.
After talking to local club members it seems the popular approach is to make the two nose stripes just wide enough so the outside edge is just outside the
oil cooler opening with the main stripes. They look pretty good that way and are easier to tape. On my car that is just about 14 1/2". Then I will taper wider towards the dash. So 6 3/4" X 2 + 1= 14 1/2" at the nose and about 17 1/2" at the dash edge, that's what I am doing.
On the other hand make them how you want!
Wayne