[GeoffR]

The font size selected with the adjustment slider is modified by the font-size style set by the publisher in the book's stylesheet. For some reason most pubishers don't like to use the default size for normal paragraphs, and change the scale to something like 0.8em, 133%, "medium", "small", etc. (each publisher is different.) If you convert the books with Calibre then the font sizes might be re-scaled during conversion. Another complication is that the ePub and KePub readers seem to have different ideas of what font size corresponds to "small", "medium", "large", etc.

(Also, the height of one line of text is not the size of the font, but is set by the line-height style. So if the final font size is 20px and the line height is 1.3em say, then each line of text will be 26px high.

Edit: One thing to be aware of if you are trying to work out the font size by counting the number of lines on the screen is that the scale used by the line spacing slider is not in em, so although the minimum line spacing on the slider is 1.0, that does not mean 1.0em. I don't know exactly what the scale factor is, but I guess it is approximately 1.3, so the minimum line spacing (on an unpatched device) is about 1.3em.

Edit2: Something I forgot that makes it even more complicated: the final line-height also depends on the line spacing built into the font, not just on the raw font size, and not all fonts have the same built-in spacing. So for example there is a huge difference between spacing of the Amasis and Kobo Nickel fonts, even at the same font-size and line-height settings the Kobo Nickel font will occupy a lot more screen space than Amasis. You can change the font's built-in spacing with tools like fontforge, but it can't be changed from the device or the book's stylesheet.

So, in short, increasing the slider one notch from the minimum does increase the base font size by 1px, but how that relates to the actual font size you see on the screen depends on the book and the font.