The female dino formula:
Start with the temp at which you can sleep at home without cover while clothed as you will be for sleeping while hiking. For me, this is 63 F
Subtract the lowest temp that you plan to encounter using the quilt / underquilt (lets say 20 F)
Divide by 10 and then subtract 1 - this will give you the number of inches of loft needed.
63 - 20 = 43 / 10 = 4.3 - 1 = 3.3" of loft
That sounds like the type of equation an engineer or physist would use.
Is that too much to ask? Girls with frikkin' lasers on their heads?
The hanger formly known as "hammock engineer".