Redesign the What if Calculator to have an entry for "EDCT time" which would automatically calculate the Latest Authorized Takeoff Time to comply with max flight time and FDP limits. Should also calculate Latest Authorized Block out time. The way it is set up now we need to calculate a taxi time that results in the EDCT and then subtract from max limits to determine LATT. Too much arithmetic. What is the point of having a computer if I need to figure it out on the back of a napkin?