I don't always use a thermometer, especially if I am just doing a few at a time.
When you heat raw plastisol, it is milky white. Even if you add color first, it is still milky. I put it in my microwave and heat it for a period of time depending on the amount of plastisol I am heating, then I take it out and mix it.
I then put it back in and heat more, then mix again. Repeat as necessary so you are not over heating but also mixing the "hot spots" into the "cool spots". Once it converts from milky to clear, you are good. You don't need to confirm it is at 350 degrees F, because it goes from milky to clear at that temperature anyway.
What gets people in trouble is not mixing the raw plastisol first to reverse settling, and not mixing well while heating. Once it converts from milky to clear (you can/should be clear and have color), you can pour or shoot. Most of us shoot at lower temperatures because some molds fill better that way. It is not a requirement.
The other thing that gets people in trouble is overheating. Once it is mixed and goes clear, it is done. Overheating will not make it convert more, it only burns it. Mixing well and often is the best preventative to yellowing plastic.
Now, reheating previously melted plastic is a little different, but that is another topic.