The correct clef minimizes ledger lines across the WHOLE passage, not at a single note

Clef choice is a passage-level optimization, not a note-level one: the correct clef for a line is whichever of the available clefs minimizes the total ledger-line burden (sum, or equivalently mean, of ledgerLineCount across every note in the passage) — not whichever clef happens to put any one note, including the first note, directly on the staff. A passage whose FIRST note sits comfortably in treble clef but whose overall tessitura centers a fourth below the staff should still be notated in a lower clef if that reduces the passage's aggregate ledger burden.

A cello line notated throughout in bass clef because its opening note is a low C, when the bulk of the line actually sits in the tenor-clef comfort zone and only touches the bass-clef comfort zone briefly at the start.

Gould, Behind Bars, Clefs chapter, states this as the governing principle for clef selection generally (not phrased as an optimization, but the intent is the passage's overall comfort, not any single note). Ross, The Art of Music Engraving, is more explicit: choose the clef that keeps the greatest number of notes ON the staff. Read, Music Notation, agrees. This is uncontroversial among the four authorities — the only real-world disagreement is about where the switchover point lies (see clef-choice-minimize-across-passage-not-single-note below for the specific comparison algorithm), not about the governing principle itself.

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