I think an engineer came on once and did some big math equation showing what size an uncowled engine should have on it for racers. Been a while but that could have been the rule of thumb. If not then I haven't heard of it before. Some people do use a spinner with cooling in thought or air penatration but the spinner is just used so the electric starter has a nice place to grab onto when starting the engine and they really give a cowled engine that clean look. You can also use spinner nuts instead of a real spinner for a clean look. I'm using perhaps one of the ugliest spinners in the world {unless it's a warbird} just so I had enough weight to get a CG and had something for the starter to hang onto. Because I can't see to operate a ruler correctly I had to cut the cowl I made and it made the spinner look even dumber but I will make a new cowl when I get the time, maybe even mold in this big spinner? anyway, different types of spinners and nuts shown.