While this documentation tells how to add a custom top level menu, it adds to the last. Say, I want to add a new custom menu "My Menu" and position it right before "Administer" menu. How would I do this?
Pretty sure you need to GET
weight of the "Administer" menu and then based on that you can SET the
weight for "My Menu" - in the civicrm_navigation table. like in Drupal I think you'll be ok having two items with the same weight - no need to renumbering them all!