URL: https://clagnut.com/sandbox/font-size-adjust-ex.html
A wonderfully useful calculator to understand the aspect value of a font.
You upload a font: src() (URL or local filename), select the font-style and font-weight. Then it calculates the aspect value for your font! Super, super exciting tool (been wanting and have searched for something like it before).
The font-size-adjust CSS3 property allows you to specify the aspect value for a font. Browsers that support this property will adjust the x-height of a fallback font to match that of your desired font. By using this property you can ensure that font metrics are more consistent throughout a font-stack.
Pro-tip...View Source then copy/paste into your own HTML your computer. It finally worked for me after I did that.