Did you let it learn in its entirety? The light quit flashing? You will never see the same temp as the liquid being on the pot itself, Only close. On a presto I would not put it near the heat source IMO. Are you using a ssr relay? These things will keep learning, I suggest you teach it 1/2 full, this way it is under a few degrees full, and over a few degrees at low level. Teach it with plastisol, your wasting your time with anything else. You can use the same gallon till it is brown if you need to wont hurt anything. You need to adjust the puf value as well, Against a candy thermometer etc. My best guess as i have did about 40 of these so far, is your true temp will be off +10 -10, Just the heat sink compound alone will alter temp.