The amount of cleaning media needed for cleaning the printing forme is determined with regard to the level of coverage with ink andor image carrier material of a printing surface andor from image data about the printing surface. A first measurement beam is directed at the stage grating at a first angle, the first measurement beam being at a first wavelength. The wetting system comprises a first wetting chamber connected to the first end, a second wetting chamber connected to the second end, and a liquid-retaining structure positioned in the second wetting chamber. The method achieves this by ensuring that each of the P sampling points is visited only once. A corresponding apparatus and design structure are also described. A bias circuit biases the transistors to turn on at positive base voltages.