Any amount down
I sail western Lake Erie and a river that feeds it - shallow to 3'-4' in some places. On my deck I've put 5 red electrical-tape stubs designating where the block belongs with the board fully up (22" draft), with it fully down (6' draft), and three midway positions (about 3', 4' and 5' draft). I sail with it in different positions depending on where I am on the river or bay. On the lake it's usually all the way down.As George says, the centerboard mainly prevents lee slippage when sailing. The water ballast does the most for leveling the boat. Motor with the centerboard up - I do very often when the outboard is running. With the centerboard up the boat tends to fishtail when the outboard is swung sharply in one direction or the other. When the board down a sharp motion of the outboard tends to rotate the boat around the centerboard - that same slippage prevention.