How to calculate percent error for absolute zero? In chemistry lab, we did an experiment with water of various temperatures to extrapolate a value for absolute zero based on experimental pressure values for water at different temperatures. Naturally, the data is a little off -- even though absolute zero is $0 K$, my extrapolated value is about $15 K$. 

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.
It sounds like this is one of those canned labs that ask you to do standard things, even when they do not apply.

As you noticed, the percent-error formula needs you to use a value in the denominator.  If you use 0K in the denominator, you get a divide by zero.  If you use your 15K extrapolation, the percent error will always be 100%, no matter what the error actually was!

In such cases, I would look at the percent error with respect to your data.  Why?  Well, because somebody said you need to calculate a percent error and you might as well use a real datapoint rather than divide by 0 or some other silly result.  If you choose your coldest datapoint, you can say something about the percent error of the extrapolation process at the least.  Thus, if your coldest temperature recorded was 2C, you could say "My extrapolation suggests that absolute zero is 260C below my coldest recorded value.  The actual absolute zero is 275C below my coldest recorded value.  This gives me a 5.7% error on my measure of absolute zero relative to my coldest recorded value."