I started using a hot-plate several years ago - probably after reading about it here. I got tired of the hokey-pokey pre-heat method on 6C moulds, where you cast in one then two, then three cavities, until all six throw good bullets. By the time I got up to six cavities, it was time to refill the pot. I stick a TC from a cheap meter in one cavity and make a mental note of at what temp YOUR moulds cast well.
Having worked in industrial process heating for several years, I can't just "warm" something or just "heat" something - I have to be able to measure it. Not that this is THE way, but it's an obsession based on solving process heating issues AND I have the stuff to do it with. If it's not your core competency, and what works for you - do THAT.
My pot is controlled with a PID controller, because I was too cheap to buy a thermometer, while I had PID controllers and TCs sitting around. When I started using a hot-plate to pre-heat my moulds, I just set a cheap Craftsman meter beside it with a k-type thermocouple plugged into it, and stuck the hot junction of the TC into one of the cavities so it touched the bottom of the cavity. I don't even know if it is accurate, but when it reads 386F, the mould will cast good bullets on the first pour, if everything else in OK.
If you have even a cheap meter with a TC jack, you can measure and record the optimum temp of your mould. It does not have to be accurate, or calibrated, or even agree with the TC in your pot, because it's RELATIVE. If YOUR cheap meter and TC reads 392F when it casts well, then 392F is YOUR optimum.
It's a cheap and easy way to achieve good results without spending a bunch of money. Lots of people have cheap meters with a temp function. If one does NOT, they are easy to acquire and can be useful for sorting out other problems with the household system of sorting out issues with your vehicle, lawnmower, log-skidder, coffee pot, .....