Change calc of xAvgCharWidth to minimize false positives
Change calc of xAvgCharWidth to minimize false positives

ported from Lasse's https://github.com/googlefonts/fontbakery/pull/1429/files
(issue #1428)

googlefonts/fontbakery,moyogo/fontbakery,googlefonts/fontbakery,graphicore/fontbakery,graphicore/fontbakery,moyogo/fontbakery,moyogo/fontbakery,graphicore/fontbakery,googlefonts/fontbakery