Something to keep in mind about that 21.75" datum... that was specifically for a 6 PPC. Not necessarily a constant for everything - though its pretty darned close.

Chris's OBT theory, in conjunction with OCW testing and software like QuickLoad can provide some impressive results. The only problem is... using QL correctly is almost an art unto itself. Certain parameters need to be tweaked to match the actual results you're getting at the range in terms of velocity, or else the results that QL predicts will be skewed. Case in point: the QL data for N150 and RE-17 in the last version I had bore almost no resemblance to reality, and had to be heavily massaged to get it to spit out anything approaching what I was actually getting for MV. At the time I didn't have a laptop that would run QL... and being able to tweak it on the range while testing over a chrono helps a lot (IIRC, thats how Chris used it). But by the time you get done screwing around with all of that... you could just about have a load worked up conventionally anyways, and guess what? You'll probably be pretty close to an OBT/OCW load anyways. It's still fun to play with, but lets just say some of the glamour has worn off for me.