Best load" depends on its intended use. For targets, it is whatever is most accurate. So, if 20 grs shoots the ball or minie most accurately then it is the best load for target work.
Was talking to a fellow at Ben Avery who was using a 2 band Enfield rifle. He loaded it with 50 grains FFF and every now and then his minies would keyhole. I suggested a reduced charge or that his minies, which were purchased and lubed with beeswax and tallow were not pure lead and that the skirts were being blown out.