Using Two Sleeping Bags in Colder Weather

So, if you want to get a close estimate of the combined temperature rating when two sleeping bags are used together, it looks like this: x-(70-y)2 =z. Tonight I will use a two bag system for a 10-13 degree night. So, I have a 30 degree bag and a 20 degree bag. With x being the lower temperature rating of the two bags (mine are 20 and 30) and y being the higher of the two. And so x is the 20 degree bag, y is the 30 degree bag and z is our answer. The formula goes like this, 20-(70-30)2=0. My set up should be good to around zero depending on several other variables, but it is a good estimate.