Pipe fall (drop) calculator
Fall = slope × run. Pick the slope your code and your inspector require for the pipe size; we don't reproduce code tables.
Water-heater recovery calculator
How many gallons per hour a tank-type heater can heat: GPH = (input BTU/hr × efficiency) ÷ (8.33 lb per gallon × temperature rise °F). A BTU is the heat that raises one pound of water by 1°F.
Read this first. These are textbook estimates for study and planning. Use the efficiency from the heater's label. For electric, 1 kW = 3,412 BTU/hr, and elements are close to 100% efficient. Real recovery also depends on tank losses and the manufacturer's rating. Your code, the manufacturer and the inspector have the final say.