A flag on a ledgered note must clear every ledger line it passes near

An eighth-note-or-shorter note far enough from the staff to sit on ledger lines still carries its flag at the stem's far end, well clear of the staff — but on notes just barely into ledger territory (1-2 ledger lines), a flag whose curve swings back toward the notehead side can visually clip the nearest ledger line if the stem is not lengthened enough. The standard fix is the same stem-lengthening rule that already applies to all ledgered notes (see ledger-line-stem-length-extension below): a sufficiently long stem keeps the flag's curve entirely clear of every ledger line beneath/above it.

A single eighth note one ledger line above the treble staff (e.g. A5) with a stem length insufficient for the flag's lowest curve point to clear the ledger line beneath the notehead — the flag visually touches or crosses the ledger stroke.

Ross, The Art of Music Engraving, Flags chapter, notes flag clearance from ledger lines as one of the concrete reasons stem length on ledgered notes is non-negotiable (as opposed to a purely cosmetic lengthening). Gould does not treat this as a separate rule from stem length, and this chapter follows her in treating it as a corollary rather than an independent constraint.

This rule is about where marks land on the page, so it can only be judged after engraving. Collision and spacing faults live here.