50 degrees sounds right.

Insulation at the floor level helps with comfort and energy use. Climate of course varies but it is nearly always warm at 48 inches below grade (in the US). Being "below frost level" would mean that it never gets below 32 - not that you should use 32.

The challenges with slab temperatures are more common with at grade slabs - then the edges get really cold. At 48 inches down, it is pretty constant.

Now - what to do with those 48 inches of walls below grade - that is a challenge. Insulated? Outside of concrete or inside? Still functional? What temp to use there? Certainly not air temp at 40 inches down but gradually getting closer as it comes to the surface.

You should consider some insulation at the floor level - 50 degrees adds up given the large area. It is commonly one of the larger losses as it is not insulated. Doesn't effect sizing that much because it is so constant - but it does effect energy use - again because it is so constant. Represents heat loss even on a warm day.

What R value are you using for the slab?

I'm using the R value of the slab at about 0.5 and no insulation under it.