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