Some years ago I attempted to understand the differences between fps and cup. My conclusion was they both measure pressure but are as different as torque and horsepower. Personally I would not trust the above formula to be safely accurate across the various cartridge configurations.

Jim

BE STRONG IN THE LORD, AND IN HIS MIGHTY POWER. ~ Ephesians 6:10

Socialism is a philosophy of failure,
the creed of ignorance,
and the gospel of envy,
its inherent virtue is the equal sharing of misery.
--Winston Churchill