How about a tool, command line or standalone app, that let's you easily compile subset webfonts for different language or charset support, together with some metadata adjustments etc. This could for example be used in the backend of indie foundries letting customers download file size conscious webfonts tailored to the customers' need, or provide on-the-fly webfonts with small footprint based on a language selection.
Great to hear it's useful to you! In fact, the Wordpress plugin internally wraps the specimenTools library Dave mentioned above, so for anybody wanting to code their own solution I can highly recommend it as a basis to start hacking. My plugin has the obvious detriment (or advantage) of being platform specific to Wordpress There's a bunch of ideas for improvements on my list for the Wordpress plugin, but if you find anything in particular (esp. in regards to Persian, which I have zero experience with) feel free to message me here or open a ticket, so we can implement it for the benefit of all users.
You might be referring to proportions rather than widths, i.e. what relation the widths of different letters have, in relation to each other, and in relation to their height.
Based on the image you pasted I suggest you look into resources relating to roman inscriptional lettering, or capitalis monumentalis. You'll find many resources as to how the square base form of capital letters relates to classical proportions. Later neoclassical thinkers have, of course, revived this idea, for example Dürer or the Romain du roi, which are highly geometric in their underlying structure and construction.
If you are looking for printed matter to study, I can recommend The Eternal Letter, which contains a lot of different essays and writing that all centres around roman inscriptional lettering and its present descendants and implications - although it is naturally not a treatment of proportions, exclusively.
When you go to Info > Features > calt you are likely to find the problem around the definitions that have to do with "s.beg_commaaccent" - either it's missing from the font, or renamed, or something the line just before or after is. If you are automatically generating the feature, refresh it. My first bet would be that a glyph of that name does not exit in the font - maybe a new update to Glyphs made it less tolerant to this particular error, or introduced some naming update that not makes your OT feature's referenced name redundant.
Also the Glyphs Forums are usually a very good place to ask this kind of software specific problems.