You can measure the x-height with the rulers in inDesign, but there are so many variables basing leading on some formula is a waste of time IMO. Trust your eyes! And, obviously, you need to read what you’re setting. If the lines are too close, finding where you left off when you get to a new line might be hard. Does the ascenders and descenders touch? Does the page evoke the “feel” you’re after? And so on …

I agree with Frode.
Read it, adjust it, read it again.
Try to use something else then Lorem Ipsum when you need filler text.
Use something you are actually interested in reading, this way you will
be reading a bigger portion of the text rather than just a couple
of lines.

Back in the days of Postscript Type 1, a non-expert could look up values such as the x-height in the AFM, which was a simple text file. Of course, the usefulness of this depended upon that parameter having been correctly set in the font, which it often wasn’t.

Nowadays, you’d pretty much have to crack open the font to get this data. The conversion to points is just a matter of dividing the value by the em unit setting in the font (generally 1000 for CFF/OTF and 2048 for TTF/OTF) and then multiplying the resulting decimal by the point size.

Or you could just measure the darn thing, as Frode suggests.

But I agree, using the x-height as a basis for determining your baseline grid seems to me to be full of potential to go horribly wrong, if you don’t already have a good eye for this sort of thing.

A simple way to determine the x-height - in InDesign: type a few x's, separated by a return (so there are x's on TWO lines. Give these a typesize as required and a Leading that is the same as the typesize. Zoom in as far as you can. Now lessen the Leading until the extremes of the x's touch (you may have to use smaller units than full points). The x-height is the Typesize minus the value of the Leading.