Most of us didn't know either, tut--pre-Internet. Picked up in bits and pieces, here and there, over a very long time. I have both Kennett's series of proof articles, which usually incorporated much of what Engelhardt had done previously (plus updates), plus the original Engelhardt articles. In this case, I'm not quite sure why Kennett and Engelhardt don't agree on the standard proof, but the difference is only 800 psi which is not all that significant.