the only
adjustable downhaul that is commercially available - to my knowledge - is the North XT mast base
but even that one you need to stop , perhaps jump in the water, etc (
i don't have one - too $$$)
so, like the others here, i make a judgement call at rigging and set it to what looks like it will work - do not use much variation here
it does not need a lot
Barry Spanier was working on an adjustable downhaul a while ago - and may be continuing
adjustable outhaul on the other hand has many benefits for not much cost
i have had it make the difference between planing and not planing - in light wind conditions
use it on my big boom and big sails, but racers seem to like it all the time
when wind is good and steady, does one really need it - not !