"In my experience a line height of 1.5 x the base font size is a great place to start, and is usually right on target." You start with your base font size, you

"In my experience a line height of 1.5 x the base font size is a great place to start, and is usually right on target." You start with your base font size, you choose an appropriate line-height for that (based largely on designerly notions like visual weight, column width, contrast, intuition, etc.) then that lineheight (in pixels) becomes your rhythm. The idea is that elements like headings, short quotes, possibly images are less sensitive to line-height than body copy, so it's okay to fudge them a little bit to keep the overall rhythm intact.