thanks for that link AFLM. When it comes to bag ratings I look more at the upper end of the rating anyway. Learned that lesson with our current mummy bags with the low end of the scale at 0F and ended up with a cold night's sleep at just 15F. That said tho, we've made some improvements to our cold weather sleep system so I'd almost bet we would sleep warm and cozy if we had to do that temp again.