Don't. Say something like "I want to keep x% of the company". Thus if you end up getting more funding than you initially expected (happens often) you'll essentially get a higher val.
http://www.feld.com/blog/archives/cat_term_sheet.html
http://www.startupcompanylawyer.com/?s=term+sheet
http://www.paulgraham.com/guidetoinvestors.html
http://www.skmurphy.com/blog/2007/12/01/three-points-about-s...