At this moment we don't have an automated tool for adding/calculating tabs onto an arc. We'll definitely have something for arc tabs available in an upcoming release.
However you can "manually" place a tab on an arc with the commands: Trim (using Circle), and Add Drill Hit. Go to menu: Edit > Trim > Using circle. Enter the diameter of the tab opening you wish to have and then pick the location (on the arc). The tab opening will be created. Next, if you need mouse-bit drills they can be added using Add Drill Hit, https://support.numericalinnovations.com/support/discussions/topics/14000002516