Draw a representation of your curved plate (viewed from the side) on a sheet of paper. Make sure the dimensions of the plate are accurate.
Draw a straight line up from the center of the plate. This line will be referred to as "Line A."
Mark the point on your plate from which you wish to calculate the plate deflection. This point will be referred to as "Point B."
Draw a straight line from Point B to Line A. You should now have two intersecting lines above your plate. The point where they intersect will be referred to as "Point C."
Measure the distance from Point B to Point C. This number will be referred to as "Y." Measure the distance from the center of the plate to Point C. This number will be referred to as "X."
Divide X by Y, and use a calculator to find the cosine of the product. This number is your angle of deflection.