title: Calculating percentage error from R-squared error
body: <p>I created a ML model with Scikit Learn and Python. I calculated R-Squared error. Is there a way to convert this error to percentage error?
  For example, If my true values are 100 and 50, and predicted values are 90 and 40, my average percentage error is 15%, because error for first prediction is 10% and error for second prediction is 20%.
  Is there a way to calculate percentage error *(average percentage error) based on the value that I get for R-squared?</p>