600 fp *20 oz= 12000 ci.
A lot of 15* bags have about 3" of single layer loft.
What are the dimensions of the bag you propose to make? As long as the bag has around about 4000 ci of area you should be ok.
It also depends on whether all of the down is on top of you (the optimal place for it, like a quilt) or if you are sleeping on top of some of it (like a sleeping bag, where the down underneath you is compressed and does no good).
A little more information might be helpful so we can get a better estimate of what exactly you have in mind.