[Bill Edgeworth]

This is not a simple task. You will also have to figure out the mass/inertia properties of the rocker arm. This will vary widely with design and ratio.
You will also have to know the approximate cylinder pressure at the time of valve opening to figure out the additional pressure required to overcome the force exerted on the face of the valve by the combustion gases. Some of the higher end CAD systems are capable of doing most of the work but you will still have to draw a solid model of the rocker arm