Subtlety and detail are fundamental to what makes musical instruments special, and worth dedicating a life's practice to, for designer, maker, player and listener alike. However, research into digital musical instrument (DMI) design tools and processes have so far mainly focused on high-level conceptual concerns and low-level technical abstractions, leaving subtlety and detail underexplored and undervalued. These nuances, and the processes they result from, cannot be fully articulated in words alone, yet they largely define an instrument's quality, and it is therefore important to understand how they come to be. We introduce a scale-based ontology that divides design details into three levels - macro, meso and micro - and we present a literature review of DMI design from the perspective of this ontology. Finally we extrapolate the ontology to consider its utility in broader contexts, and consider future directions.