I hear you! Zero to fifteen is a wide number line!
In short, runovers of number lines begin in cell 3 (with the initial line of the number line starting in cell 1), with a tick mark beginning the runover line; no continuation indicator should be used.
You probably already know, but please let me state: line graphics (NOT braille dots) should be used for all number lines in a 2nd grade transcription.
Those guidelines are from the BANA 2022 Guidelines and Standards for Tactile Graphics, which have been approved by the BANA Board and are now being carefully prepared for general distribution.
Please let us know if you need anything more or different!