Changing polygon material UV

I want to use polygons to apply tire mark decals since polygons are rendered on top of paintedlines. However unlike with aprons, with polygon materials I don’t see a way to set the UV, so I’ve been drawing them in weird shapes where the first point is the UV origin and then I kind of want to capture the part of the decal I want. But then if I want to rotate the material, which you CAN do, with every degree of rotation it completely randomizes the UV so your carefully drawn shape is useless.

Is there an easier way to do this, or at least any predictable rules I should be aware of?