In chemistry lab, we did an experiment with water of various temperatures to extrapolate an experimental value for absolute zero. Naturally, the data is a little off -- even though absolute zero is $0 K$, I got $15 K$.

One of the questions in the post lab asks us to calculate percent error, and I'm a little confused. I thought (and this post confirms) that percent error was calculated by dividing by the expected or "actual" value. In this case, however, the actual value is $0$, and you can't divide by zero.

How do you calculate percent error for absolute zero?

Using Celsius values doesn't really make much sense to me, because the Celsius scale is not absolute; $-273.15$ is essentially an arbitrary number.