What recommendation specifies the point at which a nutrient is likely to become toxic?

There is no such recommendation. There is what is called "LD50", which is the approximate dose that would cause death in 50% of the people who took that much, but nutrients are rarely evaluated like that for toxicity. Vitamins that can be overdosed on are the fat-soluble vitamins (A, D, E, K). The rest of the vitamins have virtually no dangerous dose. For any vitamin, there is a label which shows recommended dose.