To put it another way, 1000mg equals 1000ppm in 1 litre of water. Or 40ppm in 25 litres of water, which is more the concentration the formulas here are aiming for.
You can maximize yield by making sure the cathode and anode are properly positioned. For example: If you have a clockwise water flow, you could put the cathode at 6 o'clock and the anode at 9 o'clock, requiring liberated gold ions to have to travel a 3/4th rotation before they could plate onto the cathode.
There is otherwise none to very little loss. Evaporation of water just concentrates the remaining solution.