Right, so say you decide to max str -1 (so 98 or less) - you would get -16 (but yes, -18 if you get 99 str). Since it gives +20 to begin with, that's still going to give you +4 crit (+2 if you got 99 luk) - and an additional 16 luk, yielding 5 crit.
So total, if you go with 98 str you should get +9 crit, right? Or is there another factor I don't know about here? Not a bad thing to have, imho.