The heuristic I use is to note the number of degrees bellow 20c the bags are rated at and sum them. Apex 67 is ~11, so 9 plus my 400g quilt say -4 so 24 +11= -15. In practice I wouldn't take it past -10. Very tempted to combine rsbtr new 7d and see if I can get ~200g of 900+FP down to make a cutting edge stitch thru liner bag. Though I have a cumulus 150 quilt 370g so probably only 100g to be saved.