I don't think there is an equation per se, though Ackley's book may have something in there somewhere.
In my opinion, if you can't fill the case with powder as a standard round, then providing more volume is near the point of diminishing returns at the outset. I like improved cartridges, but I am not that anxious to try the 17 Rem Improved for that simple reason.
Another issue may be the feeding of the long, skinny case with the sharper shoulder.
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StevenD
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