Originally Posted by roger64
Could you give an example? How to assess exactly the height of lowercase letters? (real smallcpas and fake ones). Is this something other than the font-size?
Yes, the font-size is usually the height of uppercase letters, or something similar. Depending on how the font is designed, the actual letters may be smaller or larger than their "frame" (which is what the font-size actually controls).
But for lowercase letters, every font has a different look and proportion. The only good way of getting the right size for smallcaps is by visual judgement, and you can't know which font the book will be read with!
If we select fonts from the same family, can we be confident that the same percentage (or em?) will apply for both?* Shall we use em preferably than %?
If with the same family you mean the same name and only varying in their bold/italic character, then yes, their proportions should be all the same (assuming it's seriously designed). If they're just "similar" fonts, their proportions would be similar, but not the same.
About using em or %, they're equivalent, I believe.
I guess you could use "font-size: 1ex" (the ex unit is supposed to be approximately the height of lowercase letters), but I don't think it is correctly defined in most fonts, and I wouldn't rely on it being correctly used by rendering engines... I haven't really tried it, though.