I have met this question, and also asked clients to come up with an answer for it in connection with emergency shutdown valves.
As indicated by the three good answers above, you have to approach this by a 'guesstimate', taking all variables into considerartion.
The first question is calculation for what purpose, and consequences if the estimation is too high or two low.
Next, very often, a double consideration/calculation will do: probably not higher than contra most possibly not lower than, and an avarage.
Your considerations should contain the data (as figueres) of your calculations and modifications (example : buried pipeline reducing leak in estimated area (xxx square) at mean water pressure ( yy bar) with ( zz backpressure) and (aa percentage reduced area) - or whatever depending on method, enabling later control or repetition of the estimate.