I get, fairly consistently, 24 lines/page from epubs with "line-height: 1.2" for <p, <div, <td and <h\d, plus "widows: 0" & "orphans: 0", in the CSS.
Without those CSS entries for line-height, and with "readingLineHeight=1.2" in the .conf file, I get 23 lines/page. Explain that.
Where I don't consistently get 24 lines/page is for "long" paragraphs, that is, any paragraph of more than about 300 words. The html renderer on the Kobo Touch, presumably ADE, usually decides to throw a "page-break" before a "long" paragraph and, for a "longer" paragraph, throws further "page-breaks" in the middle of the paragraph. (The Kobo Original did something similar for paragraphs of more than about 500 words.)
The worst case is a "longer" paragraph at the start of a chapter: the successive screen "pages" display:
1. Chapter Title lines (2. 24 lines 3. 24 lines 4. 4 lines)...
It can look quite awful, especially where the first page of a chapter displays only the heading.
I get around the problem by inserting a "paragraph-break" (</p><p class="pb">) to the left-hand margin on the next line, at the end of a sentence after no more than about 250 words in the paragraph. That works for both indented- and line-spaced-paragraphs. While not looking ideal, it does at least show that it is a formatting break and not a real paragraph break.
Does anyone have a simpler solution? Don't say, "Use shorter paragraphs"; it's not my text. ADE seems to ignore all "page-break-...: avoid" directives in the CSS.