I put an ounce (weight) of "trace mineral" salt in my guy's daily feed. Eg, 99.9999% salt with just enough iron oxide to make it red. It's salt and it was only $5 at TSC for a 50lb bag, so economics kicked in. He'll eat it with more, but I figure that gets him a good percentage of what he needs sodium-wise and he's got three different salt licks he ignores to choose from for the rest. I wouldn't want to give him too much, so I figure the ounce is enough to 'boost' him towards where he ought to be, close enough that the rest is up to him.
The exception is when we're doing endurance rides or conditioning- then there's two to four TBS of enduramax (salt with a little extra magnesium, potassium, and calcium) in his breakfast, in each of the one or two bucketsfuls of mash he might eat during and after the ride, and in his dinner. He's a QH working in the TN heat and humidity though, so he sweats A LOT when working hard. Can't necessarily say he 'needs' the extra salt, since he's never had any 'issues' but it makes me feel better and he does well with it, so I figure it is most likely at least causing no harm.