Instead of a grid, which I find restrictive for some projects, I just referred to the sketch. However for more precision I would measure how far the left bottom corner of a house or a tobacco barn was away from the bottom edge and the right side edge, this helps create points of precision. I did this on each panel with the horizon line, tree lines etc.
For example if the three line was 1.8 inches from the bottom of the small sketch, it would be 18 inches from the bottom perimeter of the mural.