I grabbed that 70% number because it's the lowest fill that ignites reliably with some powders. I have now gone back and used Hodgdon's load data in QuickLOAD and found that a 20% increase in burn rate amounts to about 10% reduction in charge weight with that powder in .223 going from 50 grain bullets to 77 grain bullets. So, if you multiply the Hodgdon H335 starting load for your bullet weight by 0.9, that should put you into safe territory if the powder is what it is claimed to be. The likelihood is that will be wimpy and leave a lot of unburned powder behind, but better safe than sorry. You work up from there.
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