Crop. Measure. Square.
Yesterday I finished enough of this painting to figure out where it ends. That is a literal "ends". I work on a big piece of primed linen that I tack to the wall. I have a sketch to start out, but paintings have a way of extending their borders and changing dimensions - including going off-square! Before I can order a permanent stretcher, I have to visualize the edges of the piece and square it off. I order my museum stretchers from a custom stretcher maker in California, so it takes a while.
Once stretched, I'll complete the painting - which will be about 70" x 48".
I took a snapshot of the painting on the wall, and then tried several different cropping scenarios out in my computer to see how I liked it best. This was the one I decided on.