Proper line-height for paragraphs
You want to divide your line-height by your font-size to equal close to 1.48. Smashing magazine has a formula that you can use to get as close as you can to 1.48. 1.48 is a commonly recommended value in classic typographic books.
line height (pixels) ÷ paragraph font size (pixels) = 1.48
Also, If you know your body font size you can use this formula to find out the line height and the line length.
font size x 1.5 = (line height) x 27.8 = (line length)
In the past I have always eyed the line height and used what I thought was readable. This is important to use line height, because if the line height is to short than then the lines of text will blend in together making it difficult to read your content.
I have used both formulas and they are pretty accurate.