The present invention relates to a method as well as a blood treatment unit for the implementation of this method for the best possible attainment of a predetermined substitution target during the ultrafiltration of blood. According to the invention, a method and a blood treatment unit for the implementation of the method is provided, wherein it is determined by detection of pressure measurement values in the blood treatment unit whether the calculated substitution target is achieved at the end of the dialysis session or not and in case of the result that the calculated substitution target cannot be achieved with the current dialysis parameters, an adjustment of the flow rate of blood, dialysate and/or substituate is performed in order to achieve the substitution target for a given treatment time in an optimal way or modify the treatment time for the given substitution target as little as possible.
