Dave, I think Big John's referring to the screw hole alignment. The holes on the flange are not equidistant, so they will require lining up with the under ring. IIRC, the screw pattern is not an equilateral pentagon although it appears to be.
John, I think you could use a 1/4" wide strip of tin about 4-5" long, fold a hair-pin turn into one end, maybe 3/8" fold, and use that gap as a clamp. Maybe use two of them to keep the under ring level as you start 2 screws. Then slide tin strips out and tighten screws some, eventually, fitting the rest of the screws.
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