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.
