I'll probably take crap for this, but I never used a thermostat. When the plastic is ready, it's ready.........and you'll learn pretty quick when that is.
Not sure what type of plastic you're using, but I used MF and that stuff poured like a gem rather I wanted the colors to sort of run together or just a straight laminate. For a laminate, I would wait about 20 to 30 seconds before adding the top pour, but you don't wanna wait too long or they won't connect properly and you'll have gaps in the worm. Dangit......there's a term for that, but I can't think of the name right now. Maybe someone can jump in and save me. LOL.
I would just keep my hot plate between medium and high at all times and just keep an eye on the cup you're warming up. I worked with cups almost exclusively and I would add everything to cold plastic first. Your color, glitter, hi-lite......whatever's going in there worked best with cold plastisol. When heating your cup, you want the plastic to be at a nice thin pourable stream........kind of like synthetic oil. From that point, you want to keep it just about there without burning it.
Once you get the hang of it, you can do split veins as well, especially with MF. Pouring that second color will actually force the vein to the sides of the mold, giving it that "split" effect. Really cool stuff!! Oh and just to be clear, I'm sure other brands will work just fine. Many of those brands were not available when I poured, so I can't say either way, but MF was my go-to brand.
Your worms looks really nice, especially for a first run!! Most of the answers you're looking for you will stumble on yourself as you continue to pour. Just keep at it!!