This is a big pet-peeve of mine.
Designers who measure pixel-by-pixel spacing between lines of text or use the leading number set in their photoshop file for their style guides. It's a very common theme that I see from time-to-time that results in angry designers who complain about developers sense of aesthetics and in retort, developers who claim they are just "following the style guide".
This can all be avoided folks! Either the designer needs to convert their numbers for line-height the right way or developers need to ask how the designer came up with their spacing numbers and convert it themselves.
Here are the rules:
1. Photoshop uses what they call "leading" to measure the distance between lines.
2. CSS/HTML uses "line-height" to establish the distance between lines.
3. Leading is the distance between the bottom of a line of text and the top of the line of text underneath it
4. Line-height is the distance between half of the leading above a line and half the leading below it.
5. Here's a great illustration of the aforementioned rules: http://www.w3.org/Talks/2008/0911-CSS-Amsterdam/line-height.png
6. Therefore the formula is: Line-height = Font-size + Leading/2
7. However, if leading is set to "auto", the actual leading number is 120% of the font size.
If your font size in your PSD file is 10px and your leading is set to 20px. Then you should put 20px for line-height in your style guides.
If your font size in your PSD file is 10px and your leading is set to auto. Then you should put 22px for line-height in your style guides.