You have to make the cut using the tubing cutter after using the form and trim die so you have enough space to use the tubing cutter and not worry about the shoulder taper. A tubing cutter took many revolutions and even very slowly and gently increasing the pressure resulted (for me anyway) in a slighyly crimped cut that wasn't cleanly addressed with my particluar final trimmer.
So, I'm with Snipecatcher and did exactly the same thing (abandoned the tubing cutter and went to the Harbor Freight mini chop saw).
I have one of the first models (the cheapest one), and let me warn you - it uses the equivilent of a 12v. toy motor. It will cut brass - but you have to go slowly, and because of the clearances it has, you'll have to rotate each piece to make the cut complete.