My boom is 11'6, and I use a 3-leg system. The upper line (1/4" dbl braid) is 17' long, attaches to the mast 23' above the boom (P=34'), and ends in a nylon thimble (A). The aft leg is 13' of 1/4" dbl braid, attaches to the boom 2' fwd of the black bank with a snap hook at the aft end, goes through thimble A and ends with thimble B. A piece of shock cord (am upgrading to 5/16") attaches 3' fwd of the snap hook, goes through thimble B, and then down to the boom 3'9 aft of the mast. It has no cleats or means for adjustment.
To stow the system, I unclip the snap hook from the boom and snap it to thimble B. I then pull it fwd to the mast and hook it to something convenient like a cleat or a reefing hook. The tension in the shock cord keeps things in place just fine.
I only rig the system when lowering the main. By the time I furl the main, the lazy jacks are back at the mast.