I wouldn't use the screw, I'd use the actual spot on the plug in back of the cluster. Or is that what you mean? I had to do it a couple times...and when I pulled my cluster again to splice the oil & volt gauges in and reinstalled it, my tach wasn't working. I just had to pull the cluster again, wiggle the tach wire and make sure it was positioned right, and when I put it all back together it was working again.
Anyways I just took the wire, stripped some rubber off, doubled the stripped part over itself and stuffed it into the correct spot in the plug then plugged the plug back into the back of the gauge. I remember it taking a couple times reinstalling it before it finally worked.
Edit- you can see in my "splicing into main cluster" photo where I put the tach wire. It's the thicker blue one coming out of the plug.