On old Earth a geothermal heating unit pulls from a pretty consistent 55 degrees. Hyperborea has a slightly cooler sun and a completely different "core" dynamic. Not being a geologist or scientist I would use a slightly cooler 50-55 degrees. Wear your coat in the dungeon, but still not as bad as the top of Mt. Vhuurmithadon!
“How can I wear the harness of toil
And sweat at the daily round,
While in my soul forever
The drums of Pictdom sound?”