Plug heat range is one of a dozen parameters for engine design. The proper range of plug was determined by the engine designer when the engine was first built. It's not a good idea to change it very much from that value unless one is modifying an engine significantly. The plug must heat up to the correct temp to burn the mixture, and not leave deposits, but still remain cool enough not to glow which can cause pre-ignition or detonation.
Best bet is to start with the mfg heat range, insure your mixture is set properly, and run the engine at modest speeds for 2 hours. Remove the plugs and 'read' them. For a carb engine, the plug should be tan or light chocolate color. For a FI engine it will be light tan or cream color. Anything other than this is going to be too cold(dark brown) or too hot(off white). Carb setting, timing, compression ratio, 2 or 4 stroke all play into this.
