I just did the math, & the diagonal of your first triangle is about 6.36”. The way to determine this is: A squared + B squared = C squared. Using the inches shown on your mat, 4.5(2) + 4.5(2) = 40.5. The square root of that is 6.36”. So my first suggestion seems to be wrong because you’re using the base of the 2nd triangle, which needs to be almost 2” longer than the base of the first. The diagonal of the 2nd is longer than its base. So ignore my first advice! Again, I’d need to try it out. If I were to try it, I think I’d cut the strips 6.5” long, cut off that first triangle, set into the desired position & sew, squaring it afterwards. I hope I didn’t totally throw you off with my first mistaken post! Yikes! I’m sorry, I thought I was awake, but apparently I’m not.