As far as I can tell, most of the ration balancers are pretty similar in terms of what they deliver nutrition-wise; it's more region-dependent which one will be easiest to get. I think the Purina product is just fine, if that's what's easiest for you to find. Chewy also carries some things.
Right, NSC is sugar plus starch (it stands for "non-structural carbohydrates" so it isn't counting fiber) - Triple Crown doesn't break the two down in its analyses. So yes, the Purina is a bit higher than the Triple Crown Balancer - but is very similar to the Triple Crown Gold Balancer which I think is around 16% NSC. Either way, since you're feeding such a small amount, these differences work out to only a couple of grams in what the horse actually eats, so an analogy would be like you eating one extra bite of apple with your lunch.
Definitely feed by weight, not volume. What I do for convenience is to weigh things out on a kitchen scale (one that is sensitive to the ounce is fine for this sort of thing) and figure out what volume equals a certain weight, then I can just go by volume when I dole out the feed, I don't weigh out each meal because that would make me crazy. Rowan's serving of balancer is a heaping half-cup scoop but every product will have a different volume per unit weight since some pellets are larger, or denser, or whatever - but this is just to give you an idea, it's a small amount. If you come up with needing a 1-quart scoop of it then check your math.
I think the hardest thing with this feed stuff is getting used to translating the label information into what is actually going into your horse. The label is just what's in the feed, not what's in your horse's bucket. The minerals are all expressed in parts per million so to find out for example how many milligrams of copper is in your horse's serving, you have to calculate that. The nutrients expressed as percentages are easier but you still have to do some arithmetic to figure out what your horse is actually eating. These balancers are really pretty great though - if you just feed it by the label directions you should be pretty well covered in most situations (unless you have some weird hay or your horse is not eating hay for the main part of his diet).
Right, NSC is sugar plus starch (it stands for "non-structural carbohydrates" so it isn't counting fiber) - Triple Crown doesn't break the two down in its analyses. So yes, the Purina is a bit higher than the Triple Crown Balancer - but is very similar to the Triple Crown Gold Balancer which I think is around 16% NSC. Either way, since you're feeding such a small amount, these differences work out to only a couple of grams in what the horse actually eats, so an analogy would be like you eating one extra bite of apple with your lunch.
Definitely feed by weight, not volume. What I do for convenience is to weigh things out on a kitchen scale (one that is sensitive to the ounce is fine for this sort of thing) and figure out what volume equals a certain weight, then I can just go by volume when I dole out the feed, I don't weigh out each meal because that would make me crazy. Rowan's serving of balancer is a heaping half-cup scoop but every product will have a different volume per unit weight since some pellets are larger, or denser, or whatever - but this is just to give you an idea, it's a small amount. If you come up with needing a 1-quart scoop of it then check your math.
I think the hardest thing with this feed stuff is getting used to translating the label information into what is actually going into your horse. The label is just what's in the feed, not what's in your horse's bucket. The minerals are all expressed in parts per million so to find out for example how many milligrams of copper is in your horse's serving, you have to calculate that. The nutrients expressed as percentages are easier but you still have to do some arithmetic to figure out what your horse is actually eating. These balancers are really pretty great though - if you just feed it by the label directions you should be pretty well covered in most situations (unless you have some weird hay or your horse is not eating hay for the main part of his diet).