Or just looking for a way to evaluate the fit of the line to your data or, conversely, the fit of your data to the line? If it's the second one, you can choose the "options" tab in the add trendline dialog, and choose to have excel display the equation of the trendline and the r-squared value. R-squared is a measure of the goodness of fit of the line; it's approximate interpretation is that it describes the percentage of the variation in the data that is explained by the equation.
It is calculated by summing the squared differences from the line. The closer it is to 1. This is a more standard and accepted way of determining the "regularity" of the data as you put itthan just summing the un-squared differences.
If you want to calculate residuals the distance of each data point from the predicted value at that point you can use Excels "Slope" and "intercept" functions to determine the equation, then apply these to the x-values to get the predicted "y" values, then subtract the actual from the predicted value.
You can plot the residuals to help determine if your data might be better described using something other than a linear regression, and can test these residuals to see if they are normally distributed another measure of goodness-of-fit of the line.
Hope this helps.