Note spacing must follow a sub-linear (logarithmic) duration curve, not linear beats × width

The horizontal space allotted to a note must grow more slowly than its duration: doubling a note's duration should roughly multiply its space by ~1.5-1.7x, not 2x. This is the Wanske/Gourlay proportional-spacing model LilyPond implements explicitly (space = base + k·log2(duration/shortest-duration-on-the-line)) and that Ross's engraving spacing tables approximate empirically (his tables compress the ratio between a half note and an eighth note to well under 4:1 even though the beat ratio is 4:1).

Ross, The Art of Music Engraving, spacing chapter (tabular, non-doubling space-to-duration ratios); LilyPond Notation Reference, 'Horizontal spacing' (documents the log2-based Wanske/Gourlay formula directly — the most exactly implementable citation, since the source is public); Dorico's 'note spacing' preference exposes a similar sub-linear ratio, default non-1:1; Gould, Behind Bars, discusses the principle qualitatively without a formula. NOTE: authorities agree on the qualitative sub-linear shape but not on the exact exponent/base — LilyPond's constants are tuned for its own engine and are a reasonable Gradus starting point, not a universal constant.

This rule can be decided from pitches, durations and voices — no layout required. A validator can check it before anything is drawn.