I can't quote you R-values, but 2 ridgerests should be pretty good. I use a closed cell pad (about equal to a ridgerest, probably) with an older ultralight thermarest (about like a prolite3) on top. the closed cell is full length, the thermarest 3/4 length. This works pretty well, and is a combination that I've heard quite a few people use. The 2 ridgerests should be as warm or warmer than this combination. The list of R-values you found may have numbers for the pads I use. Ridgerests are very light, but bulky, so if you are in doubt, take 3, if you can find a way to strap them onto your pack. Better to have more insulation below you than not enough.