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As you may know already, surplus powder varies in burn rates from lot to lot. There are no data tables for surplus powders, only recommendations for start charges. Your 872 may be a little faster or slower than other lots of it. You have to work up surplus powder EVERY time you switch to a different lot of powder. That's why buying enough of it to last a while is best so you don't have to do load workups every time you buy more powder from a different lot number. For common 50 BMG powders such as IMR 5010-WC870-AA8700-WC860-AA8600-WC872 the start charges are typically:
647-650 grain bullets--start at 210 grains
690-700 grain bullets--start at 205 grains
800 grain bullets--start at 195 grains
Since it's a big volume of powder usually you work up from start charges using 3-5 grain increments. Just a guess but I'd think that 215-218 grains would be near maximum with your combination. Check your brass for signs of case stretching with the circular stretch mark inside the case. Make sure primers are seated flush or below. Have fun but be very careful!
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