Rather than going by temperature, I actually go by time and volume. Once the cider gets down to about ⅛th its initial volume (½ gallon takes me about 3 hours at a low simmer), that’s when I take it off the heat. You can also start testing how thick the syrup is getting on a plate every 15 min for the last hour or so of boiling - you want it to have the consistency of runny honey.