That's the way I have recently started doing it, set up the mast first. Then you can use an offset turned just right to give you the distance you need for your mounting straps or strut.
That said, we still have some where we have to mount the panel first. Today we did a 400 amp overhead. Mounted the panel, then I got out my brand new Fat Max laser plumb bob that I bought for laying out hi hats and plumbed for the 3" mast. Came out perfect.
I am happy to report we did another service today, used the Fat Max after the loadcenter was mounted, it came out perfect again. I cut out a little cardboard circle to fit inside the hub, then marked it dead center and lined up the laser plumb bob with the mark.
I heard people call that tool a mast-out tool.2 inch hole-saw on a 2 foot bit wrapped with a 2 inch ridid.The rigid revolved freely,in other words you could hold on to the rigid pipe and pull the trigger on the driil and the hole-saw would spin but not the pipe........thought I saw them in parts houses later but not sure
