Height in inches is not as meaningful as height in nodes. Top after the 5th node and then again when the side growth has 4-5 nodes on it.
You should not start in a large pot. Start in a SMALL pot and re-pot frequently. FYI for next time. The plant will still finish rootbound very often even if the pot is large, if it was put in a large pot too young.
You should plan to flower at about 18" high instead of 3 feet because the plants will double in height during flower, adn a 400HPS does not have all that great canopy penetration.