CVRMSE

In the tuning contract, at the far right, you see "deviations" which indicates how far the fit line is from the bill. Then at the bottom of the table on the far right, you will see X+-Y, where X and Y are numbers. X is the mean bias error, which ASHRAE Guideline 14 says should be < 0.05. (Actually, you can force that to be 0, in the Project Preferences form), and the Y stands for CVRMSE, or Coefficient of Variation of the Root Mean Square of the Error. ASHRAE 14 says that the CVRMSE should be <25% for energy, and <35% for demand. The CVRMSE is almost the Root Mean Square of the deviations. It is roughly a measure of the average distance of the points from the fit line. Many see the CVRMSE as being more important than the R2 value, which is really all most of us were concerned with in the 90s.

If you need more help on this, refer to a statistics text book.