The stock downhaul is 8:1, and this is the class-legal limit. You can cascade to 16:1 by having the stock downhaul pull on two tails running up to cheek blocks on the tackboard of the sail, then down to jam cleats right at the foot of the mast. This will not be legal, and will produce a great deal more power than is needed for the I-20 carbon mast (real nice w/120-lb crew on my heavy-masted P-19, tho!). A better option on your boat is to use the same theory, but reduce the main downhaul to 4:1. The line, following it from the starboard edge of the boat, will run to the exit block on the starboard side of the mast, up to a loose (unmounted to anything) single bullet block which has about a 2-3' tail attached to it, down to a fixed single bullet block (I believe it's on the boom gooseneck, but correct me if I'm wrong), back up the port side of the main to another loose bullet with a tail, down to the port exit block, and out to the edge of the hull. The tails on the loose bullet blocks are led through cheek blocks mounted back-to-back to the tack board (from aft running forward) and down to jam cleats on either side of the mast. Old salts will tie the tails together in a nice square knot in front of the mast.
hope this helps, if not I can take pix over the weekend. I needed an excuse to rig the boat anyway, gotta finish the new spin rig.

seeya at the Ides of March, or maybe, just maybe, I'll be able to bust loose and make the 6-hour haul to Deep South.

sail fast