I would set and store a base font-weight, line-height, and font-size as a variable and work off those.  Generally, `16px` aka 1em is considered good.  

I'd recommend a modular scale.  Check out these links for more info:
http://alistapart.com/article/more-meaningful-typography
http://typecast.com/blog/a-more-modern-scale-for-web-typography

Remember that you can do math in SASS, i.e. `$base_font x 3` or `$base_font / 2`