Use the energy balance principle.
Specifically, compare the distance moved at the input and output of your linkage (for a small movement, since clamp linkages often have a ratio which varies with stroke).
Since energy in = energy out, [force in × distance in] = [force out × distance out], therefore the ratio of forces is the inverse of the ratio of distances.
If the distance moved at the output becomes zero, theoretically the force at the output is infinite and you've achieved a toggle clamp.
