There are two gages at the bottom of the dashboard that are for estimating Accuracy and Surface Finish. Because we are measuring the movement of the cutter with the tap-test we can predict at what speeds will deliver the highest accuracy or lowest surface location error (SLE). If the tooth arrives too early or too late in its back and forth vibration cycle, that speed will result varying tooth impacts that do not match the programmed tool path.
Surface finish is determined by the waviness left by cutter. This comes from the cutter's dynamic displacement (CDD). The greater the depth of the waviness the worse the surface finish, as illustrated by these two images:
The closer to the bottom that the line reaches, the less the depth of the waviness.
Both Accuracy (SLE) and Surface Finish (CDD) are speed dependent and don't alway correlate to the stable speeds on the RPM dial. Resonate frequencies often occur within a stable lobe. You can be very stable, but not very accurate. You can be very stable and leave a poor surface finish. If you lower the depth of cut, typically radially, you will find a stable speed for a finishing pass that will also yield a good surface finish or accuracy for a finishing pass.