A well drilling system is provided that includes a choke manifold and a controller. The choke manifold includes at least one choke valve. The choke valve is actuable between fully open and closed choke positions. The choke valve has a Cv value for each choke position. The controller is in communication with the choke valve and a non-transitory memory storing instructions. The instructions relate Cv values to choke positions for the choke valve. The instructions when executed cause the controller to: a) determine a difference in pressure (ΔP); b) input or determine a density value; c) input or determine a Q value; d) determine a first Cv value using the ΔP, the density value, and the Q value; and e) actuate the choke valve to a first choke position associated with the first Cv value.