On the DHW tab (detailed mode in Wizard), there is a Sink/Shower% control that allows you to set the total sink and shower water usage as a percentage of the standard usage for the number of bedrooms in the home. You can view and edit the standard assumptions on the Setup/Usage&Costs tab. You can model an improvement that reduces usage by setting the improved % to be less than the base %. The savings for this measure are listed separately in the improvement tables under the heading Reduce Water Use.