Width os/2 value calibrated on xAvgCharWidth

[davelab6]

https://twitter.com/TiroTypeworks/status/1101699667773992960 :

Thought: should a variable font Width axis scale be set by font makers relative to the OS/2 table xAvgCharWidth value, given that this is the only data point for what width means in a proportionally spaced font?

http://typedrawers.com/discussion/comment/39605/#Comment_39605