212F is not 212F, reading material here:
(More heat (btus) = more boil off)
You don't want to use a lid either, you need a rolling boil to blow off nasty compounds like the precursor to DMS. When boiling you really want two things:
1) The lowest heat setting that maintains a rolling boil. (Typically, unless trying to rapidly reduce volume)
2) Predictability, so that you can try to always boil from 7.5g down to 6.0g, or whatever your preferred volumes are.