Let the sashing begin!

I finished making all my Granny Square blocks, and this is my probable layout. Now I'm adding the black sashing around each block. I already goofed by sewing on my first horizontal sashing strip according to what I calculated to be the final width measurement. The smarter thing would have been to actually measure my finished row and then cut and sew on my strip. After fighting and stretching that strip as I sewed it on, I unsewed it, then measured and - yep - it was a good 5/8" too short. So I wasted a lot of time, but I'm back on the right track!