A method improves the performance of a jetting assembly in an apparatus for jetting droplets of viscous medium on the fly onto a substrate. The jetting assembly is releasably mounted in the apparatus and includes a nozzle, an ejection mechanism connected to the nozzle, and a viscous medium container connected to the eject mechanism. Information relating to an XY-position and a Z-position is obtained together with information regarding the exit velocity a droplet of viscous medium has when it exits the nozzle. The information is used as a basis for adjusting a trigger position of the nozzle for the subsequent jetting of droplets. A jetting assembly also includes a storage device arranged to receive and hold calibration information of properties of the assembly obtained during calibration measurements.