I hope I understand your request. You want to calculate via an equation how much an installed valve would leak under a specific condition?
I would think that how much a valve is leaking depends on the particular valve, such as the current condition of the seats. Also, you did not specify type of valve. I would think that different types of valves would leak differently as well. Hence, I don't think it is possible to calculate it.
The only way I can think of to determine how much your particular valve is leading is to meaure it.
Of course, once you do, and put it back in service, it will continue to deteriorate, and the leakage rates will again change with it.