Example methods, apparatus and articles of manufacture to calibrate valve-mounted instruments, such as a position controllers and/or a position transmitter, are described. A disclosed example apparatus to calibrate a valve assembly including a valve, an actuator and a position sensor includes an interface to receive a valve position value and to receive a position sensor sensitivity value, and an endpoint estimator to compute a first estimated value corresponding to an expected fully-open position of the valve based on the position sensor signal and the first and second values, and to compute a second estimated value corresponding to an expected fully-closed position of the valve based on the position sensor signal and the first and second received values, wherein the first and second estimated values are computed without repositioning the valve between computation of the first and second estimated values.