Right on BP. Absolutely insulate under any aprons or sidewalks.
The 2' down and/or 4' out kind of cover the same area.
You need to imaging the heat that is escaping from under the slab as opposed to the cold going down into the ground. Cold is a relative term for lack of heat. Anyway.......
The heat will escape at ROUGHLY a 45* angle out from under the concrete. Down or out will both stop this loss of heat.
Yards & such I tend to go out & slightly down. For buildings, slabs, parking lots, etc. I like to go down so there isn't the loose gravel issues.
