Originally Posted by oneclick

There's a change in the resistance-to-effort that you can feel when these things set. What happens is that you stress the metal past yield, and after that point the stress-strain slope changes, it feels easier. Then as the collar squishes up against the flange it goes up again. (After that the resistance falls to zero.)

This is much easier to tell if you have a lever setter.

If the OP did not get a few extra rivnuts with which to practice he might back up and do that.