He said 300 because there is NO real magical temp to go by, as long as the engine is giving off a smoke trail and not bogging(too lean) or burbling(too rich) it WILL be fine but when too hot, the fuel mixture ignites before top dead center and cause detonation (very bad), I've had an engine hit the 320ish mark at the end of a hellacoius main with no problems, long story short, dont tune to a temp... it almost ALWAYS leads to a poorly tuned engine, fouled plugs, and smoked engines, tune by sound, power and look for smoke, then if you must, for a reference, check the temp and youll be fine.