I thought you had to add 7/8". I would use Anita's paper method first. I use the Square in a Square ruler so I don't have to do the math!
You are right about 7/8" - when you cut a square on the diagonal into two triangles. But I believe she is using the method of sewing squares to a square and lopping off the excess. It's a good way if you want to keep things extra stable.
Anita