Design the dynamics for a Domestic Water Pump Calculator

To Create an online calculator available for Clients to use to determine the size of water pump & pressure tank that they would need for their home. We would like a solution that would be very simple for the ‘lay man’ to use that would require them inputting the number of points of use e.g. washrooms, kitchens, other minor terminations as well as the level of the building ie. Ground floor, 2 storey etc. Though the interface must be simple, in place of accurate inputs we wish to use acceptable substitute values, e.g. what is the average amount of water consumption that should be catered for, for each Washroom, furthermore how the height of the floor affects the demand (flow/head) on that floor, furthermore how the amount of residents relates to the overall picture. Based on the outputs certain output recommendations would need to be made regarding the sizing of the pump and tank. So this project requires Validating and confirming the necessary variables, acceptable values & methods for calculations and dynamics to ensure that the concept is ready to be carried to the stage where a designer can get involved to create the App for our Website. There are numerous Calculators like these online of varying technicality but we wish to stay on the simple side and are open to recommendations.