It's also related to the shimming of the engine. Ie. with higher nitro and less compression (more shimms) you can use hotter plug. To me it's easier to use the hot or ultrahot plugs for many as the engines are not so picky on the LSN and they iddle better and takeoff more constantly even if you are slightly off with the LSN. GO works just nicely with colder plugs as well but you need to tune them slightly differently for apparent reasons