Get a piece of clear tubing the length you want to measure. stake it at each end of the patio with the ends up and fill it with water. Make sure both ends are above the rest of the tubing and that their is some unfilled tube at both ends. measure down from the water level to the patio at each end. Subtract. You have a very accurate measure of the total drop. Divide by the length to get the fall per foot.
Doesn't take into account dips and humps if that is part of your problem.
Frank