Is there a rule book saying a quilt has to be 20F or 40F rated? Just look at quilts on the market, and calculate how much warmth = fill-power you can pack with what you have.
That's about the quantity of fill-power in the newest JRB partial UQ and more than the fillpower of an HG 40F Phoenix. If HG made a 40F Burrow in the short size or RAB did, the way WM does for some of their bags, that's just short of how much FP they'd include. (So fill the the part of the footbox your feet would crush with synth.)