A method and apparatus as provided for determining, for an ink to be deposited on a substrate by a halftone inkjet printing process, a nominal coverage value (the ‘ink restriction value’), that corresponds to an amount of ink sufficient to fully cover the area of the substrate to be printed. This determination is effected by measuring the reflectance of the printed substrate for a range of nominal coverage values, and then using an automatic processing arrangement to determine, from the change of measured reflectance with nominal coverage value, the nominal coverage value at which continuous tone behaviour commences.