Hi, I am off today to build a 15 x 30. I will get a few detailed photos for you. I only use a string on one side and a center point as a guide. From there I space with the bottom rails and connecting straps.

With a tape measure I make sure the distance apart is the same at the front and rear of the braces. I also check the entire brace assembly for square before backfilling.

I hope I can post more this evening, Dennis