ok, this is my remark copied from a different forum thread - I said: "Your statement reminds me that I was reading about combined (bag) temp ratings and found an article regarding a formula used to calculate your "system" and what rating one could expect from the use of two bags together. The formula looks something like this: If you have (for example only) a 20° and a 35° bag together, you would subtract the liner bag's rating from a value of 70. Then, halve that number (we arrive at 35/2 = 17, roughly) and subtract from the outer bag. So, 20 degree bag/quilt with a liner bag of 35 degrees, gives you a 3 degree rating. So far this seems reasonably correct, I have used a combo of WM Ultralite/highlite down to 7 degrees without supplimental clothing. Does this sound reasonable?"
so i was off by 4 degrees, but close.