From Jardine and consistent with other sources:
ETR = 100 - (40*Th)
where ETR is est temp rating and Th is thickness.

So for a 0 degree estimated temperature rating:
0 = 100 - (40*Th)
(-100) = -(40*Th)
100 = 40*Th (divide both sides by -1)
100)/40 = Th
2.5 = Th
So with 2.5 inches of loft it theoretically should be good to around 0 degrees.

As Yogi Berra said, in theory, theory and practice are the same. In practice they are not.