How much is easy.
Take a bunch of empty 45ACP cases. Pull rounds. Dump the powder of each individual pull into an empty 45AC case. When you have 10-12 of them, take them over to your scale, along with a pen and paper, and weigh the contents of each one. If the charge is consistent, then you know how much of that powder is supposed to go in that specific load, anyway.
Once you have pulled the ammo, you have removed any possibility of squibs and double charges. That's my major concern with OPR's, anyway. The ammo might be "hot" or "weak" but if you make up a few and try them out, you will know. And you can add extra caution by backing off and working up, for your own specific guns.
As long as it's the same powder and charge weight coming out of the cases, I am not going to throw out a half pound of perfectly good powder.