Generally speaking, does there exist an "optimal" line height as a proportion of how high the text is, for maximum readability?
1.2 ems is standard accepted for best readability. Often it gets to be a bit bigger on websites.
longer lines of text can benefit from more spacing as well (easier to find the next line as you're reading)
For the block text I personally use this simple rule:
Vertical space between the lines (put in lowercase) should be ca. 180% of the height of the lowercase letters.
To illustrate the rule, I copy paste the letter "o" and fit it between the lines:
So the optimal spacing IMO is when I can fit slightly less than two whole letters between the lines.
However, one should be aware of the fact that the number (line height) is not even related to the optical line height (lowercase height), but normally related to the so-called em-height, so it is better just to look at the actual result and adjust accordingly.