All the documentation I've found on PWF basements never shows insulation under the slab floor. The allowed back fill heights are always referenced to the bottom of the bottom plate of the wall which coincidentally is the bottom of the slab.  If under slab insulation is installed after the perimeter PWF walls are in place, the slab will be raised by the thickness of the insulation.  With under-slab insulation  is the backfill height the height above the slab bottom? or the bottom of the bottom plate? 

Backfill height is the difference in height of the exterior and interior ground levels. When there is an interior slab, the interior distance is measured from the top of that slab.

I've answered my question by actually doing the design according to "Permanent Wood Foundation Design Specification" To accommodate a raised floor one uses the crawl space formulas. I have my floor slab raised by 2 3/8" of foam and found a tiny, almost negligible, difference in stud stress. If one doesn't exactly follow the prescriptive guidelines with all their limitations actual design is needed and not all that difficult. FWIW my 2500 sq ft walk out basement with full 8 foot ceilings in the basement has 9 foot 1 inch back fill (measured as the guide requires from the top of the bottom plate) on the uphill side, my garage floor has NO steps to the sub floor, and I need doubled 2x8s on 16" centers to allow that plus I had to align the floor framing (I joist) with the studs (the second top plate is flush with the top of the joists and the sub floor extends over the PWF) The soil loads with a 9 foot back fill are substantial and require a fully blocked floor diaphragm and interior basement shear walls.