I'm beginning to wonder if you may have air getting back into the system somehow, are all your pipes/unions either new or airtight and fit for purpose?
Also where exactly is this nut, are you inferring that you've attached it between the clutch fork in the bell housing and the end of the slave cylinder piston?
There should be absolutely no need for it! If you've had cause to do this then something is very, very wrong and I'm beginning to wonder if you're going to have to strip it out and start again.
Clutch forks can bend, that would necessitate shimming out with a nut as you have to bring the pedal back up, they're not that expensive to replace