These modified cases will not hold a bullet, as there's no neck tension. In use, you adjust the rod to gently push the bullet into the lands, tighten the adjustment screw on the rod, extract the tool and measure using a dial or digital caliper measure what would be the overall length of the bullet and case with the bullet inserted into the insert, and with the rod extended to the base of the bullet. In calibers with such freebore as the 204, it can be very difficult to measure unless you have three hands. Try laying the assembly on the bench on a flat rag when trying to measure if the bullet is out of the case; it prevents everything from slipping about.
Good luck, Jim....you're on the right track!

On crimping.....agree with Joe O.