If you have a power panel it is a simple matter and one Iuse to get the best power from my engines. Choose your prop and fuel , start the engine and get it warmed up, set your needles to best peak. Now add glow heat with the panel, check rpms if the rpms go up with heat added you can use a hotter plug if it goes down your plug is either hot enough or too hot.
Ialways use the hottest plug Ican in order to keep the idle and transition smooth.
Another thing about idle bar plugs. Some port designs cause the intake charge to be directed at the glow plug , this may have its advantages but it causes the plug to be washed with cold fuel at low rpm trying to "put out the flame" so to speak. An Idle bar plug can help prevent this and improve the idle as well as the transition. Although this is more common with baffle ported engines there are modern engines that also exhibit the problem.
Merlin Plugs seems to have the most comprehensive line Ihave seen.