This is how I think I should adjust a slightly noisy roller rocker - tighten down the nut until the push rod does not rotate easily, then tighten another 1/2 turn and lock it. Sound right?
If it's a hydraulic lifter, you can also adjust it with the engine running. It can be messy, but that's actually how I did it for many years. You don't want to rev the engine with the valve covers off. But at idle, you likely won't get oil squirting and it's actually a very accurate way to adjust them. Make sure the engine's warm first so it'll idle nicely off the choke and you'll be fine.
If it's a hydraulic lifter, you can also adjust it with the engine running. It can be messy, but that's actually how I did it for many years. You don't want to rev the engine with the valve covers off. But at idle, you likely won't get oil squirting and it's actually a very accurate way to adjust them. Make sure the engine's warm first so it'll idle nicely off the choke and you'll be fine.