screening it would be a pain in the ass. but your right, I've never tried to take a LB of butter out of a pot full of trim. I've never screened any of my trim, I just let it dry all the way out, and make cooking oil instead, I find it more universal that butter, at least, for the shit I can find to bake. i really have no idea how much to use.....
but lets talk it out.... people usually use around a oz of good bud for a LB of butter, and generally I've heard about 2-3 oz's trim per oz of good bud. so I would take about 3 ozs of trim, and put all the keif you get out of that into the butter.