Forwardmost relative to the direction of the force vector. Extend the force vector to the floor, if it intersects within the contact points of the legs, it’s stable. If it intersects outside, the stability will depend on the force exerted by the mass of the object. This is really just a free body diagram problem. Project those two forces (tipover force and gravitational force due to mass) to the ground, and draw a line between them. The point where that line intersects the axis of the legs nearest to the tipover force vector is your fulcrum point. Simple calculation from there.