I have a paragraph tag that I defined elsewhere with a line height of 15px, and I have another paragraph tag further down the page where I want to make the line height around 10
I've noticed in both Firefox and Chrome that if you set the HTML5 doctype there's a minimum line-height for inline elements. For block elements you can set the line-height to whatever you want, even make the lines overlap.
If you don't set the HTML5 doctype, there's no minimum line-height for either block or inline elements.