How much downhaul you need to use depends on the sail.

My first A-class sail was really kind on the downwind, just let out some mainsheet and everything was ok. My latest mainsail goes like a bullet downwind but is really tricky to handle in higher winds.

Applying a lot of downhaul will help you control the power from the main on the downwind, but if you pitchpole hard enough with a lot of downhaul applied you will break the mast. The A-class regattas usually abandon racing when you reach the point where you need to apply a lot of downhaul on the downwind to survive.

I would recommed a three step rule, first tighten the outhaul, then release the traveller all the way out and if you are still overpowered apply some dowhaul but don't pull it all the way down.

/hakan