The length gauge (the rod) is split where the threads are. Slide a screwdriver in that split and carefully "open" the
two threaded sides a tiny bit. That'll make it hard to screw in.
Then I take a piece of leather in the jaws of a pliers and hold the bottom of the length gauge,(it's smaller there, not the neck size, not the point, just where it gets smaller) and screw it in too the cutter head. I want it too turn hard, you can watch how close the shoulder of the rod gets to the cutter.
Then it's just a matter of trimming a case and measure,,repeat,, till I get it to where I want.
I got the Lee Zip Trim unit, it makes handling cases alot easier, I check trim length every 3-4 cases, but it doesn't change.
I found for me, the key is to not "horse" the cutter into the case, just slide it gentle like and let it cut till it the gauge rod lands on the shell holder.