How much of the total data is not? Well, we already have a measure for that. We have the squared error of the line. This tells us the square of the distances from each point to our line. So it is exactly this measure. It tells us how much of the total variation is not described by the regression line. So if you want to know what percentage of the total variation is not described by the regression line, you would just say, this is the total, it would just be the squared error, the squared error of the line, because this is the total variation not described by the regression line, divided by the total variation.