I have made Rhubarb, black current, damson and apple jelly
I don't use a thermometer. I boil it for about 10 mins depending on the jam. I keep a few saucers in the fridge then put a little jam on one and put it back into the fridge for a few mins then bring it out and tilt it to the side, if it wrinkles your jam is at setting point. The juice of a lemon can help increase the pectin levels for some jam so it sets easier. My favourite to make and eat is strawberry