I mix my raw plastic, color and anything else in 4 ounce batches. One minute on high with a 1000 watt mw. Stir at 45 seconds, again at the minute, shoot the temp, and add maybe 15 secs more if needed. Different colors a different additions to the plastic can change the initial cook time a few secs so best to check. Re-heats depend on the temp of the plastic and re-melts can be done in 45-50 secs, stirring after 30 secs and every ten secs until the plastic is able to be shot.
All of this will come quickly to you. Until you have a solid understanding of the plastic and cooking times I suggest you get a note book and write down every step you take/make from the pouring to the shot so you have a specific reference in case you bump heads with a problem. If you have good notes you can generally see where things took the turn and get back on track from there.