How tight exactly should the Anycubic FEP be tensioned?

If the film is too tightly tensioned, your prints won’t peel off properly and may stick to the build plate, too loosely tensioned and it will have layer lines or other unwanted things.
It should sound like a snare drum when you lightly tap with a finger. You can use an audio analyser to check, factory tensioned FEP film (about 127 to 150 microns thick) is around 350Hz, but anything between 250Hz to 375Hz is fine.

Anycubic video: