For calculation the shape of the seal would be important. For a flat sheat calculation can be rather easy but for other typical seal shapes it might not.
Another point would be if the seal is in the direct line of the force or it is situated in a groove which limits deformation.
I wanted to add that typically a rubber seal is not operated at low deformations and due to the typical s-shape of the stress strain curve the use of Youngs Modulus would lead to too low calculated strain values.