Nice idea and a rather catchy realization! Yet the precision becomes way too tedious rather quickly - I would recommend either making a set of angles (8? 16? 32?), or adding some extra control helper (like "shift" for 45-degree rotation)? Or if you really want to stick with all the possible angles - maybe add a laser pointer? Then it won't be about shooting and waiting to see how accumulated angle errors make you miss, but rather mirror adjustment game :)