Originally Posted by LeicaFoReVer
how am I going to find and mark at identical angles?
Easy: Get a linear polarizer for the viewing filter, as linear ones work either way (circular ones stop working with the rear facing outward, as the light is then going though the circular plate first). Hold the viewing filter in front of the main polarizer, point at a wall or other area source of unpolarized light, and turn it until you find the angle of maximum brightness. Cross-check the result - if it is not near black at 90° offset to either side, there is a misoriented circular polarizer somewhere in your setup.