I think what he is suggesting is having a top off that monitors SG, so that as water evaps and sg rises the top off would kick on to fill the tank vise just the pump starting at a specific water level.
I don't know of a system out there that works on that premise but what you can do is monitor your SG as water level drops and at the SG that you want you pump to start, mark the tank. Then place your switches such that the pump kicks on and off at the marked levels. This is the only way that I know of that you could accomplish what you are trying to do, IMO.