A lean ~50-line body that presents only the method, the base value, the arithmetic steps, and a summary table, with no padding or explanations of concepts Claude already knows.

Each step gives fully concrete, copy-paste-ready arithmetic such as '81 × (2/3) = 54' and '54 × (4/3) = 72', plus a final table of exact lengths and ratios.

The four calculation steps are clearly sequenced and followed by an explicit Validation section with checkpoints that confirm values and check table-vs-step consistency; for a simple single-purpose skill this is unambiguous.

Under 50 lines with a single, self-contained task and well-organized sections (Overview, Base Values, Calculation Steps, Final Scale Lengths, Principle, Validation); no external references are needed.

Names multiple concrete actions — 'calculating pentatonic scale pipe lengths', 'tuning traditional Chinese instruments', and 'derive 宫商角徵羽 from base unit 81 using alternating 2/3 and 4/3 ratios' — matching the anchor for listing multiple specific concrete actions.

Explicitly answers both what ('Applies the three-division method...to derive 宫商角徵羽...') and when ('Use when calculating pentatonic scale pipe lengths or tuning traditional Chinese instruments'), with an explicit 'Use when' trigger clause.

Includes natural user-facing terms like 'pentatonic scale', 'pipe lengths', 'tuning traditional Chinese instruments', and the method name '三分损益法', giving good coverage of what a user needing this skill would actually say.

A highly specific niche (ancient Chinese pentatonic pipe tuning) with distinct triggers that would not plausibly fire for unrelated skills.