How are actual yield and theoretical yield determined?

The "actual yield" is, well, what you obtained. "My yield was 42mg"

Explanation:

But if the calculated "Ideal" yield was 100mg,

42mg/100mg x 100% = 42% yield.

Theoretical yield Itself is based, usually in moles (and then grams), by the limiting reagent, or the "Ideal" yield is how I learned it long ago.

Let's say 2 reactants react in a 1 to 1 Ratio; but you have 1 mole of A, and. 5

Moles of B; from this, it shows you can get 1/2 mole of product maximum. I did answer in this way because the wording was just a bit ambiguous.