You will find the Ross roll crimper to be an excellent tool. I've tried all the others, and have been rolling my own for years.
You will find the amount of hull above the overshot card, necessary for a good crimp varies a little, depending if the hulls are cut down or NEW and Skived. The difference is the thickness of the wall to be rolled. The rollover only needs to make a 180 degree turn, and any more than that is unnecessary, and may result in slight increase in pressure.
I leave about 3/16" of excess hull on 12ga. New Cheddite hulls, cutdown Federal hulls, and some New Paper Alcans I bought here.
My 28gauge hulls, with 3/4oz. of 7.5 or 8 shot, cut to 2 1/2", leave more with SG28-II wads than Fiocchi RO28 wads. I stick with hull lengths that match the chamber for best gas seal. I don't want to trim hulls to accommodate certain wads, so until I find a filler, I'd either have to use an extra couple overshot cards, or end up with an unnecessarily deep rollover.
I'm making it my rule of thumb is to find the correct wad for the shot/powder charge, and not vary the case length because of a compromise in components. Not easily done with the shortages we see today.
To your question about the cleanliness of the cut; yes, it matters. If you do a search, you'll find the hull trimmer jig I made, which yields a perfect edge.
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