I can't think in pounds per acre, there is just too much variation in seed size between different hybrids and lot numbers. We've been planting 25k seeds per acre since it's so dry, but if we started the season with good subsoil moisture then we'll go up to 30k. Some guys here will plant 18-20k, but the way I see it, 25k will yield more if it rains, and if it doesn't rain it won't matter how much you planted. Milo seed is cheap.
Doublecropping is a slightly different situation. I've been told that you want to plant a little heavier to limit tillering (the crop would mature more evenly), but since you're dryland and don't get much more rain in the summer than we do I would be leery of planting more than 35k. It's kind of a crapshoot; milo can compensate very well for thin stands, but can fall flat on its face if it's just a little too thick and you miss a timely rain. I think I would shoot for 30k, that's a nice round number.
I can't help with the sprocket settings as our both of our planters have hydraulic drives (and aren't Deeres). Maybe someone else will chime in. If you're lucky, since the previous owner planted milo with it last everything will be close.