A few further thoughts on number lines are below. Many thanks to members of the Tactile Graphics Committee for their input!
The revised TG Guidelines (not yet published) state that number lines are graphics and so should have a blank line before and after them. And number lines should start in cell 1 unless a single number line is the entirety of an identified item (e.g., each answer choice consists of only a number line). This is so a number line will (in most cases) fit on one line without division and be easy to find.
The revised TG Guidelines also tell us that a numeric passage is not necessary for the numbers below a number line to be transcribed without any numeric indicators. In other words, numbers below a number line don't have numeric indicators, and no numeric passage is necessary for this to be the case. Tactile Graphic Guidelines tells us that all number lines are considered to be tactile graphic representations, whether they are done with braille cells (grades 4 and up) or as line graphics (K through grade 3).
The collection of rules and guidelines that apply to a UEB Math/Science transcription is surprisingly diverse. UEB Guidelines for Technical Material (GTM) applies to most "dot level" issues. BANA's Provisional Guidance for Transcribing Mathematics in UEB applies, especially to formatting concerns not covered in GTM. Braille Formats 2016 applies to text format issues not covered in GTM or the Provisional Guidance mentioned before. Guidelines and Standards for Tactile Graphics (GSTG) applies to tactile graphic issues, including number lines. The updated version of GSTG will include the symbols appropriate for number lines in a UEB Math/Science transcription. These correspond quite closely to what is in the GTM. (The 2010 version of GSTG can tell us little or nothing about dot-level concerns in a UEB Math/Science transcription.)
Since GSTG applies to these number lines we're discussing, we can use its thorough information about how to transcribe number lines, including how to format runovers (GSTG 126.96.36.199). Namely, a continuation indicator is not shown at the end of a line; a break must occur so that a tick mark starts on the new line; and the runover is in cell 3 (with the very beginning of the number line in cell 1).
So, to revisit your original questions in brief:
- A number line should almost always be formatted in 1-3 regardless of the context/exercise material.
- According to GSTG, numbers below a number line do not get any numeric indicators, and no numeric passage is necessary for this to happen.
- There is not a required number of horizontal line indicators between an arrow or a crossing vertical line. Spacing in a number line must be proportionally accurate based on the number line's content.
- Runovers of number lines begin in cell 3, with a tick mark beginning the runover line; no continuation indicator should be used.
Phew! How is it going?