Before you start dumping supplements into your horse, you should consult your vet regarding the affectiveness of Vitamin A and rain rot. The term ration balancer befuddles me; how can a horse's rations be balanced without bloodwork.
I think you might be overthinking it
A ration balancer is meant to fill in the nutritional holes that arise from feeding a horse on forage alone (particularly on hay) without adding a bunch of extra calories. Many (most?) people rely on grain-based hard feed to fill in these nutritional holes, but grain is less than desirable for many reasons; my horse is both an easy keeper and ulcer-prone, so I avoid giving him any grain, but I still want him to get the added nutrients that he isn't getting from hay alone.
It's true that to get a perfect nutritional fit for each horse, you'd have to do a bunch of testing, not just of the horse, but also of the hay, pasture, and any feed you give. Not only that, but you'd have to repeat these tests many times throughout the year as seasons change, hay ages, etc. It's not impossible to create a highly customized diet for a horse, but it's generally impractical, which is why people rely on pre-mixed feeds (grain, ration balancer, or supplements) to provide nutrition. These pre-mixed feeds generally work well for the majority of horses, but it's up to the owner to ultimately identify any nutrition-based shortcomings and compensate for them, which may take some trial and error.