First, select "Create/Edit Performance Scales" from the menu.
(https://smf.newcastle-technology.com/index.php?action=dlattach;topic=150.0;attach=131;image)
Then, add your performance scale. You can call it anything - see "New BC" in the example below:
(https://smf.newcastle-technology.com/index.php?action=dlattach;topic=150.0;attach=133;image)
Then, hit Done at the bottom. When you go into your class settings, you will now be able to select the performance scale you created:
(https://smf.newcastle-technology.com/index.php?action=dlattach;topic=150.0;attach=43;image)