Offside should be calculated the same way as goalline tech. Use a tone in the refs ear, or a lamp somewhere, Whenever a player is in an offside position (no worries about phases, or interfering with play, simply on the pitch and closer to the opposition goal than two defenders and the ball) then the ref has a tone in his ear, he uses the presence of this tone along with his judgement about when the ball was played and whether the player was interfering with play, as those parts are subjective
Let the subjective stuff stay subjective, of course use tech for the objective stuff, that makes complete sense