I have no idea about the R values of my pads, but I know what works-a Ridgerest and original Thermo-Rest stacked together. I use full length pads, which I think is better than using 3/4 pads for sleeping on snow. I'm a cold sleeper and this combo works fine for me. My bag is a -5C (+23F) rated down bag and I've used it down to about +10F with my parka thrown on top. This works because my parka is the biggest down parka North Face makes. I sleep in my thermals (Capilene midweights) with a hat or balaclava on and sometimes my liner gloves and socks.

Another tip-eat something like chocolate or an energy bar before dozing off and have one handy in case you wake up cold. You need fuel to keep your core temp up all night.