The percent difference between a known value A and a calculated value B is:
|A - B|
% diff = --------- * 100%
A
I have primarily used this in Physics labs when doing experiments.
An Example:
Find the percent difference between 11 and 12.5, with 12.5 being the calculated value.
|11 - 12.5|
% diff = ------------- * 100%
11
% diff ~ 13.64%