Reference for Hydration Levels

There is no such reference. In the strict sense, hydration levels are only strictly defined for non-ennriched breads (those without dairy or other sources of fat).

If you want, you can calculate some kind of hydration level for enriched breads, but they become less informative. One option would be to count the whole weight of the fat/dairy as "hydration", the other option would be to count only the water proportion of the product you are adding. I would generally prefer the second one, but it has the problem of getting very misleading when the fat proportion goes up.

Both methods don't create reliable comparisons. For example, if you have a bread at 70% hydration, where all the hydration comes from 700 ml water, a bread at 70% hydration where you are using 795 ml milk, and a "bread" at 70% "hydration" when using 4.1 kg of butter (!) will be really different in their behavior. Similarly, a dough with 700 g water, or 700 g milk, or 700 g butter will not behave the same.

You will get closer to comparability if you also track the fat amount added, that makes it of course also complexer.

If you still insist on calculating something, here are the numbers for the water proportion in the liquids you asked for (needed for the "second method"):

• Butter: 17% water
• Margarine: different, depends on each brand
• Evaporated milk: 35%
• Buttermilk: 87% water
• Heavy cream: 67% water