But I still find it to be high regarding the text label. I have tested
with other fonts with the same result because the capital letters were
always shorter than the full ascent of a font.
FWIW, I put this in front of a friend who used to do UI design, and they said, roughly "you can try to get fancy with centering checkboxes on font metrics, but most people actually like "a few pixels above the baseline", as long as you don't go above the top line. Part of why this works is due to the fact that very few fonts have capital letters with descenders, so visually, you almost always get white space below the adjacent letter. If you want to get fancy, you can compute "a few" as something like '(floor (/ font-size) 4)', but most designers can find a fixed value that works the vast majority of the time". Tying this in with Eli's suggestion, this could be a configurable value, defaulting to, say, 3 or some divisor of the font size.
Hope that helps,
~Chad