Gary,
It's generally not a good idea to adjust luff tension using the halyard on a big free flying sail. This is because, if there is too much distance between the head of the sail and the halyard sheeve, the sail can begin to oscillate or "dance" back and forth. In extreme cases, such as a halyard being eased by a novice deck hand, this oscillation can broach a boat and knock it down (I have seen this!) Now, if it's only a foot or two, it's not a big deal. But it is generally best to hoist the head all the way up and deal with luff tension via the pennant. This also greatly reduces the potential for halyard chafe aloft.
I use a 2:1 purchase on the tack: a single block with becket with a pelican clip in the bail. I reeve a line that is made off to the becket up through the tack of the asymm, then back down through the sheeve, then back to the capstan on the windlass where it is made off with a tugboat hitch. For me, the 2:1 is adequate; if I am unable to tighten the luff by hand with that, then there's too much wind for my 1050 ft/2 sail, or it's sheeted too tight.
~A
