I am just a small time hobby pourer, so this might not work for you, depending on how much you plan to pour.
I have found that it's difficult for me to heat less than a cup of new plastisol without danger of overheating it. Smaller amounts just seem to go from close to scorched really fast for me.
If I'm going to use salt, I add it before I begin heating the first time, because it takes a while for the salt to get hot, too.
I also add some heat stabilizer, and any softener or hardener, before I begin the initial heating.
I heat it to 350, and then add my glitter/flake. I've found that glitter/flake affects the final color, so I stir it in first. Once it's reached 350, it never has to be reheated that hot again.
Finally I add my colorant. By the time everything is stirred in well, the plastisol is too cool to inject, so I put it back in my microwave, and reheat it to 335+-, depending on which mold I'm shooting.
Having an infrared thermometer is crucial in letting me heat my plastic without overheating it.
Remember, a microwave heats from the middle out, so stir you plastic before you take it's temp. That way you'll get a much more accurate reading.
I've found that already cooked plastic is much easier to reheat in smaller amounts. I don't know why.
You will "ruin" some plastic. There is no other way to learn. Just take comfort in knowing everyone who pours has done it, and likely more than once. It's kinda like leaving the plug out of your boat, or leaving one of the trailer straps attached when you launch. Everyone has done it.