Modern gestural interaction and motion capture technology is frequently incorporated into Digital Musical Instruments (DMIs) to enable new methods of musical expression. A major topic of interest in this domain concerns how a performer's actions are linked to the production of sound. Some DMI developers choose to design these mapping strategies themselves, while others expose this design space to performers. This work explores the latter of these scenarios, studying the user-defined mapping strategies of a group of experienced mid-air musicians chosen from a rare community of DMI practitioners. Participants are asked to design mappings for a piece of music to determine what factors influence their choices. The findings reveal novice performers spend little time reviewing mapping choices, more time practising, and design mappings that adhere to musical metaphors. Experienced performers edit mappings continuously and focus on the ergonomics of their mapping designs.