A good rough starting point for x-height ratio is about .5
But it can vary significantly.
Times Roman has an x-height ratio of about .45 and Roboto Condensed is .53
As for practical application, it's fundamentally important to senior UX or Information Designers, as it lets you determine the actual physical height of the lower case "x" — a critical measurement for usability.
Fluent reading text size for people with normal vision ranges from a low of about .2 degrees of visual angle (VA) to a high of 2 degrees. This translates to a range of 4 pts (1.4mm) to 40 pts (14mm) at standard viewing distance of 40 cm.
These numbers hold roughly true regardless of the medium — screen or print — with some variation based on reading stressors and audience group.
So minimum x-height text size for body text would be 1.4mm, but you'd be safer for most audiences with 2.5mm. As viewing distance increases, ideal VA remains constant but of course physical height needs to increase.
If you want to measure x-height ratio exactly, choose your typeface at 100 pts in your graphics app, type a lower case "x", and measure the height of the letter "x" in points with the app's measuring tool (you can use any measurement unit as long as they're the same for both text size and x-height size.