What is the exact formula in getting the measured distance if an error occur in a tape, this is about survey..?

some books explained the too short error and too long error, however some given problems and solutions are different, others solved it by: if the error is too long, they are subtracting/adding, and also too short, they are subtracting/adding, now, what is the exact solution or formula to be follow, thanks a lot for the helps thanks...