Here on ux.StackExchange they have gaps that are a bit smaller than one line height (~88%), but I’ve read a few times that 100% of the line height provides best readability because it continues the grid of the text lines. I personally find the 100% gap a bit too tall. Does the eye even process these gaps as parts of a grid?
Your content will have its own tone, rhythm and meaning. One of the main jobs of typography is to help bring this out.
As kontur stated "it requires a visual eye and is always context-dependent". While it is true that the final choice should come down to your specific example typography is quite an old art and has a lot of guidlines.
While there are no hard and fast rules, these guidelines should help in most situations:
- Try to set a baseline grid. This will help your page maintain a vertical rythm.
- As line lengths get longer or font sized get larger you should increase line heights (in typography this is known as 'leading'). This makes it easier for youe eye to scan back to the start of the next line. A line height of 1.5 times the font size is usualy a good starting point.
- Do not make your line heights (or paragraph spacing) too high or the text will appear fragmented.
- Paragraph spacing is usualy less than a single line height (often 75% - 90%). This is to compensate for the lack space taken up by descenders (there are no descenders on a blank line).
- Each new paragraph should be clearly seperate from the last but not so seperate that it looses continuation.
- Use 12 point font size as a minimum. This usualy equates to 16px but not always.
- Stop your text feeling cluttered by using enough whitespace.
- I am also a big fan of using the golden ratio for many layout solutions (not just text).
Typography is a subject that can (and does) fill whole books (and lots of them) and can take a lifetime to master. It is always worth getting a graphic designer or better yet a typographer to check what you have done - they will be able to quickly give you tips for improving your specific examples.
Some more reading for you:
- The Perfect Paragraph
- The Paragraph in Web Typography & Design
- Setting Type on the Web to a Baseline Grid
- The Optimal Text Layout is More Than Line Length
- Secret Symphony: The Ultimate Guide to Readable Web Typography (using the golden ratio in web typography)
- Typographic Design Patterns and Best Practices
The best readability that I have personally tested observed time and again is the golden ratio, keep the line height at 1.61803398875 and check out the paragraph readability. Anything more makes the UI waste a lot of space whereas anything less does not convey optimal readability