I just did the same, and built the Delta yesterday. In the build video, Josh mentioned that the angle wasn't critical - just as long as both fins were the same. So, I used the foam cut-out from between the inside of the elevons. It worked out great!
That's what I was going to do if I couldn't find the answer. Did you also notice there were 16 pages on the site and only 12 to print. I'm thinking the gauge might have been in the other pages in the past...