Residual

Beni

I am looking for a good way of calculating the residual of Pressure Correction. I think it should be very simple to calculate the mass-imbalance of all cells divided through inlet massflux. So far so good, but if I am making this the error is always approximately zero because I am extrapolating the velocities at the outletboundary and correct it then to assure global mass conservation (like in the Peric Code CAFFA)! Where is my error in my thinking, or is there another better way of calculating the residual, also for further pressure correction equations like in PISO?! Thank you for your help!