Percentage deviation from a line of best fit?

I have a set of 20 data points where roughly 15 of them are in a straight line and the results start to trail off. I need to be able to compute the percentage deviation of each point from the line of best fit that was fitted using the ezfit tool.

Is there any way that this can be done?

[darova]

There is a way: you can use polyfit() to fit your data to line