Percent difference: What to place on the denominator? I'm genuinely confused on what to put on the denominator for the formula $$\%~\mathrm{difference} = \mathrm{\left|theoretical~-~experimental\right|\over theoretical}$$ or some other similar formula with different variable names that mean the same thing.

Specifically, I have a reading on the iron content per pill written on the bottle, and I have an amount that I calculated from doing some math with the experimental data. The problem is, I know that the calculated value is experimental, but the iron content per pill isn't really theoretical because it's known. Anyway, which should I put in the denominator?
The experimental value is the one you measured/calculated yourself, from your own experiments. The theoretical value is the one that you obtained from some third party, to which you are comparing your experimental value.

So, the number on the bottle is the theoretical, and the number from your data and calculations is the experimental.