The lens you are using is great.
I use an Apo Rodagon 90 for this purpose 2 1/4 square and get good coverage.
Let me know where you find the large magnets, I can draw a diagram to show you how to set up for Murals that is no brainer, repeatable and will save you
hours of grief. I am printing right now but will try to do the drawing this weekend and post.