Bracketing on single-line staves

Is there a way to graphically change the length of a bracket so that it doesn’t stick out so much when it ends on a single-lined staff?

Welcome to the forum, UniGiCa. I’m afraid there isn’t really any direct control over this at the moment. The minimum height of the bracket for a percussion staff will always be as if for a three-line staff, and then the value of ‘Maximum number of staff lines to use minimum staff line protrusion’ on the Barlines page of Engraving Options is added on. I’ll make a note of this, and see about adding some additional control in future.