You can mark it, but most photographers still fine-tune a polarizer by looking at the scene and rotating until the effect is where they want it. A reference mark may help you learn the filter’s behavior, but it usually won’t replace visual checking.

Some polarizers already have a manufacturer’s mark, though it may not be precise enough to trust blindly. If you add your own, first make a temporary mark with pencil or tape so you can test and adjust it before making it permanent.

A safe approach is to put tape around the outside of the rotating ring only, avoiding the threaded area so the filter can still turn. To find an orientation, aim at a reflective surface such as water and rotate the filter to minimize reflections; then place your temporary mark. Once you’re satisfied, transfer a small permanent dot to the outer ring using something controlled like a toothpick and a tiny amount of nail polish or correction fluid.

In short: worth trying if it helps your workflow, but expect to keep judging the final setting by eye.