Density or reflectance targets for respective first and second marking engines of a document processing system are determined. A series of control patches is printed with the respective first and second marking engines. Relative reflectance values of each control patch printed with the first and second engines are determined. First marking engine relative reflectance error values of each control patch and second marking engine relative reflectance error values of each control patch are determined correspondingly based at least on (a) corresponding first engine relative reflectance value and target relative reflectance value and (b) corresponding second engine relative reflectance value and target relative reflectance value. Based at least on one of the first and second marking engine relative reflectance error values, at least one of the first and second engine relative reflectance targets is adjusted. Based at least on the adjusted target, an image quality control of the document processing system is improved.