Easily good down to at least -10°F
for me, although I don't know the R value: Cut-down Xtherm Max 71"x25" (20.4oz) combined with a 3/8" CCF 71"x23" (7.5oz)
on top of the Xtherm.
I tested with a GG thinlight (1/8" CCF) and it wasn't enough for me. A Ridgerest Sol was too much insulation for me, thankfully, way too bulky and weighs nearly a pound. Goldilocks hard a work.
However, in an extreme situation where I'm going to be out a long way from a trail head, the Ridgerest might be taken because it would get me through the night if the Xtherm sprang a leak.