I have a boomless sail on my Prindle Escape. The downhaul is pretty much the same as most other old style Nacra's, there are two cheek blocks, one on each side of the mast, down about half way to the mast step and a clam cleat down about 5" below them.
I hung a Harken double block off the tack so it would be easier to put on max downhaul. I run the line from the cringle, to one cheek block on the mast, back up through one of the Harkens, down to the other cheek block, back up through the other Harken, then down to the clam cleat.
I find I have to pull it pretty tight before leaving the beach then if it's really blowing, sheet the main fairly tight first and pull it some more. But I can bend that phonepole of a mast if I put my back into it and the sail will flatten out nicely.