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That's what I figured when you mentioned the wide open halos... My first (rubber grip) was similar, sharp wide open but halos on anything bright. The one I have now (metal, radioactive apparently) does not suffer from this at all.
At any rate, it is certainly working pretty well for you it seems.
As for the operation, I just sat the lens upside down, marked where I wanted to trim, unscrewed the retainer and filed it on the bench well away from the lens. But again, it is certainly working for you the way it is.