As I understand it (please someone correct me if I'm wrong), the clamping force will never be more than the force you are applying. It's just going over a shorter distance so the work input required is reducing. So if you apply 100N of force to the toggle, as you approach the toggle point, you are simply approaching 100N at the toggle output.
you can work it out really simply by just drawing shapes (I like shapes, not equations :P). You will need to pick a point in the travel of the linkages. The example I give you below is if there was 100N loaded onto the hinge, and the linkages were 20º away from being parallel to the sliding plane. Then you just draw a diagonal line 100mm long and 30º from horizontal, and draw a rectangle around it. The horizontal distance is your horizontal force on the sliding plane, in this case 94N.
I'm not sure how to work this out purely mathematically, but this gives you a rough idea.