Brad Feld venture capitalist and Managing Director Foundry Group
“I’ve found that even sophisticated entrepreneurs didn’t necessar[ily] grasp how valuation math (or “deal algebra”) worked.  Recognize that this is about the math behind the calculations, not the philosophy of valuation .
 The key trick to remember is that share price is easier to calculate with pre-money [valuation] numbers, and fraction of ownership is easier to calculate with post-money [valuation] numbers; you switch back and forth by adding or subtracting the amount of the investment. It is also important to note that the share price is the same before and after the deal .”Brad Feld, Venture Capital Deal Algebra, July 7, 2004; http://www.feld.com/wp/archives/2004/07/venture-capital-deal-algebra.html