I build my own sails, been doing so for the past 40 years.

I thought Id pass along one of my personal perceptions of the best 'options' for a mainsail .... an over-the-top leechline. Such allows leech adjustment from the base of the mast (or from back to the cockpit) as well as from the aft end of the boom.

Instead of terminating the leech line at the head of the sail, I attach a small cheek block to the headboard and apply a small sleeve along and on top of the boltrope/luff sleeve. The leechline goes into the cheekblock and 'turns' down into the sleeve. At each reefing point/cringle on the luff the leechline sleeve is partly omitted for a short distance (no sleeve at each reef cringle) so I can adjust from each luff reefing cringle; there is a jam cleat at each luff reef position and of course the tack. The jam cleats are arranged at an angle so that when pulling the leechline from the safety of the mast base, *all* jamcleats 'above' the one being used will release the leach line.

This arrangement allows me to control leech tension for all reef conditions, and more importantly - no matter how far the boom is off center, and I can still adjust the leech from the leech end if the boom is close to the boats centerline, as well as the safety of the mast base in 'stink' conditions.
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Why did I do this? A flogging/fluttering leech will soon destroy itself in 'blammo conditions' as the leech flogging/fluttering is quite destructive to the leech stitching. Once the leech stitching breaks, the sail material will soon rip and the seamed panels will/can 'split'.
Also, because too many times I found myself hanging onto the 'well outboard' boom with one hand, the other hand futzing with a leechline, one foot on the cap rail, the other 'dangling' over water. One time I slipped and found myself dangling with 2 feet 'free' over the water, hanging on with two arms ... well 'outboard'. Now, all I have to do if the boom is well outboard is to take a short walk to the mast-base / gooseneck / tack and do all the leech adjustment from the relative safety of the mast base.

FWIW To further lessen leech flogging/fluttering, which usually occurs between the headboard and the first top batten, or between the top and second from the top batten (most common site of leech flogging), I usually add a small thin, 15-18" long small 'auxiliary' battens to this leech area .... for a more stable leech and even less need to 'tweak' the leech line.