Sorry, but each time I try to explain anything in the next post you mix everything as if you were not paying attention and I have a feeling I am wasting my time. I told you that the theoretical voltage is the one you have calculated, then you propose that the error is between theoretical voltage and the calculated one - in other words, error is between the theoretical and the theoretical. Does it make any sense?

You are expected to compare MEASURED with THEORETICAL, where THEORETICAL means CALCULATED from the table data.