"There is a scale under the film pressure skate, which should be set in the center."
The skate pressure isn't calibrated, so the center position on the gauge is not necessarily the "normal" position, although that may be the case with a specific projector.
"The only time you should ever need to adjust the amount of pressure would be if you were projecting a shrunken or thinner than normal film that is shaking in the gate."
The gate pressure should be checked every show and backed off until the pressure is just enough to keep the image stable.