Originally Posted by FBinNY

You want it functionally tight. Meaning tight enough to steer without slipping.

I do this using the wheel between the knees test described above. It takes amazingly little handlebar force to steer, so if the bars resist twisting, they're tight enough.

I prefer tight enough to need some force to move, but not so much that I can see or feel the fork flex. We used to call this "race tight" because it allowed the bar to twist in a crash, to reduce damage or injury.

For the rest, read Andy's post above.