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CloseReview for Ruler Bar by Alexander Kriegisch
Rated 3 out of 5 stars
Ruler Bar works well in my environment (mostly text-only messages, others not tested yet), but I had to play around with the settings (e.g. scale of marks) quite a bit to make it exact for my font (Lucida Console, 11 px) - 135%, by the way.
I would suggest to improve the add-on in a way which enables automatic calculation of fixed font widths.
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