It's a reasonably good starting point, but bear in mind this is only half the story as you need to add to this value the deflection between the bed and frame, for the same force.
If you want a less subjective answer, then measure the force you're applying and divide by the deflection. As a rough guide, if you get at least 1000N/mm stiffness you'll probably be fine for most things, but clearly it depends on what the cutting force and accuracy requirements are.