Missing from the equation is the weight/mass of thrusters (and RCS's) needed to achieve the force. AND space to fit it, if to have it inside a hull. So then you find out how much force per KG a thruster gives. Then calculate in needed RCS weight. Oh, yes, then the size needed to fit it all, after finding the thrusters to use. Ouch, need so much space? Then maybe have to rebuild and recalculate it all.