Adjusting palm mute duration?

Couldn't find anything on this when searching -- when I add palm mute from the guitar palette (P.M. ---|in the guitar palette) and it works. Great! But I don't want the duration to be a quarter note, and I can't seem to find a way to change the duration directly. Best I've found is adding the "let ring" direction on the note I want the palm mute to end on. Am I missing something simple?