I don't think anyone can give a single "ideal" number for most nutrients.
They try to work it out by testing higher or lower levels and seeing if the chickens do better or worse. That can be used to figure out a range that seems to work, avoiding deficiency and overdoses. Some things cause trouble if you provide too much, while others cause no problems but don't give any benefit either. Salt and calcium are ones where both ends of the range matter, because it causes problems either way. Overdosing on amino acids is not really a concern, so it's just a matter of making sure there is enough.
Cost matters too. For large commercial flocks, the "ideal" is the one that gives the most eggs for the fewest dollars spent on food and care. For some backyard flocks, the "ideal" might be the one that provides the best health and long life for each chicken, even if it costs more than the one that's chosen by the commercial farmers.
And just to make it more complicated, some individual birds seem to have slightly different needs than others. For example, crooked toes can be a symptom of riboflavin deficiency in chickens. But that doesn't explain why one chick may get crooked toes (that go away with supplemental riboflavin) when other chicks eating the very same feed are fine. Sometimes this happens if the chicks are all the same breed, and even have the same parents.
I think the best anyone can really do is check what levels are known to cause problems, and choose a feed within the range that appears to work for most chickens, and change feeds if it does not work for the particular chickens you have.
There definitely are sources that tell what numbers or ranges have been found to work (although they may not all agree, since research turns up new details every now and then.) I notice that another poster has already provided a few sources.
