Originally Posted by JSWolf
Is smallcaps a percecnt of the capitol or is that again based on the font in use?
Smallcaps are the same height as the x-height. The x-height is the height of a lower case "x". They picked a specific letter since, especially with serif typefaces, the x usually has nice flat tops and bottoms, while many other letters, like c-e-m-n-o-s...often differ a bit from the x-height, usually a smidgen higher on the top of a hump like in the letter "o".
X-height is a font metric, one of many, that is inside every opentype font file.
It can't be a % of cap height since the ratio x-height/cap-height varies. Modern fonts for screen use typically have larger design for x-height than traditional print typefaces (e.g. Georgia or Verdana v. Times Roman), in an effort to increase readability.
Cap height is not necessarily the same as the value for 1em, either. An em also includes lower case ascenders (usually higher than caps), descenders, and maybe more space, so that if tight line spacing is acceptable, no leading is needed.
"Em" used to be a horizontal measurement - the width of a capital M. But that's usually pretty square anyway, and now it's used for vertical height. Anyway, that's where "em dash" (and "en dash") comes from: a dash the width of the letter.
Even if one gets caps set to x-height, the text will still not look right. They will be too light/thin, and the spacing will be off. I haven't experimented, but a bold at 0.4-0.6 em might look closer to right than a regular weight - a semi-bold probably will. You really need the smallcap glyphs, either from a separate font file or, preferably, as a feature in the original font.