An exposure pattern is computed which is used for exposing a desired pattern on a target in a charged-particle multi-beam processing apparatus so as to match a reference writing tool, possible of different type: The desired pattern is provided as a graphical representation suitable for the reference tool, such as a raster graphics, on the image area on the target. A convolution kernel is used which describes a mapping from an element of the graphical representation to a group of pixels which is centered around a nominal position of said element. A nominal exposure pattern is calculated by convolution of the graphical representation with the convolution kernel, said nominal exposure pattern being suitable to create a nominal dose distribution on the target when exposed with the processing apparatus.