Absolute error is (correct value - estimate), and relative error is (correct value - estimate)/(correct value). For example, if you estimate a distance as 500 miles and the actual value is 505 miles, your absolute error is 5 miles, and your relative error is 5/505 = 0.99%.

Approximations (linear or otherwise) are typically less accurate when they're further away from the known value. So I would expect the relative error to be greater at -0.25 than at 0.1. I didn't do the calculation, though.

- Hollywood