There is no perfect approach to automatically pin the view scale parameter to the end of your extension line.
You would need to use a “dummy” line in the view title family, although even if you added an instance parameter for length you wouldn’t have direct access to it. You would literally need to group the view scale parameter with the reference line, and align & constrain the “dummy line” as well. There would need to be a length parameter driving the length of the extension line.
And lastly, you would need to create a view titletype for each size you would like to utilize. Again there is no perfect approach for this, and I’m not sure I would recommend this approach either, but simply wanted to make you aware of one possibility:
Video Example: http://screencast.com/t/QJUmqKaz2S57
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