We present a friction model for the curved contact area between a deformable object and soft parallel gripper jaws for grasping posture estimation. We show that the assumption of a planar contact area leads to an overestimation of the frictional force and torque, which might cause the object to slip. We simulate the contact with the Finite Element Method, then compute the friction wrenches, which are fitted with two limit surface models: an ellipsoid and a convex 4th-order polynomial. Despite a slightly higher fitting error, the ellipsoid limit surface is chosen to compute the grasp quality because of its simplicity. We compare the limit surfaces of our friction model with the planar contact model and show the improved accuracy obtainable with our model. We then apply the presented model for grasping posture estimation by simulating the contact for all grasp candidates. We show a grasp quality map (quality of all grasp candidates) and the best possible grasp location for several deformable objects.