Then there seems to be a little bit of a gap on the outside boundaries as well, which is hard to tell without zooming in closely:






So if you tried to use the upper crossing curve as a cutter, it does not quite go all the way across the surface and that will make a weird messed up boundary like you're getting. You will also need to include the original side edge as a cutting object as well I think in order to get a good trim.

- Michael