Quiz numerical answer between two values

I am using quizzes to test with mathematical calculation questions.  I'd like the questions to be marked against a numerical answer I input in the question creation but, depending on how the students rounds their calculations as they progress, I need the system to accept an answer that is either +/- a certain percentage of the value I create or accept a value between two values.  For instance, if my answer to the question is 265.07, I need the system to accept any value between 264 and 266.  Is there a way to do this in the code?  Where is the code that validates the answer from a student against the answer in the question creation? Thanks.