This might be a naive question, but why are they still estimating how much oil is leaking? If they have pumped x amount of heavy mud in which temporarily capped

This might be a naive question, but why are they still estimating how much oil is leaking? If they have pumped x amount of heavy mud in which temporarily capped the leak but was subsequently blown out because of the pressure of the escaping oil, couldn't they determine the exact pressure that the oil is coming out, and if they know the size of the opening, a flow rate?