The way to decide what size template you need:
2 1/2" + 2 1/2" = 5" - (2) 1/4" seam allowances = 4 1/2"
4 1/2" + 4 1/2" = 9" - 1/2"(seam allowances) = 8 1/2"
8 1/2" + 4 1/2" = 13" - 1/2" = 12 1/2"
You can also use copy paper or any paper and cut it and sew it and iron it just like fabric. When I want to know what I will end up with for a size I cut out the squares in paper and sew them to answer my questions. It is a great way to practice a new technique too without having all the flub ups in fabric and wasting fabric.
When I needed to design the paper templates I used copy paper to make a block then took it all apart after I had cut out the wings block and used the pieces to draw my templates with a seam allowance plus a bit more for leeway.
I have cut out squares of paper and sewn them together to determine what size template I need to get the width I want in a project. My trial and error way of finding answers.
I cut out 5 squares of 2 1/4" to find out what it would look like and how it would measure width wise. I am a visual person and have to see it to make sense of it. Math only goes so far for me. Then I tried other sizes of templates til I got the finished size I wanted.