When I was in highschool cooking class (many moons ago) they taught us to mesure butter by putting some water in a large measuring cup and adding butter until the water leavel had increased by the amount you want. So if you want 1/2 cup, start with 1 cup water and add until you have 1 1/2 cups. Then pour off the water of corse! Does that make sence? Anyway it should work for any solid fat.