Drake's Equation

http://www.activemind.com/Mysterious/Topics/SETI/drake_equation.html

I am trying to make a good estimate of how many space civilizations there are, however I need your help /sci/ on giving me a good estimate for use variable.

I'd figure how long our planet has been around devided by how long we have been able to get into space? So (earth yrs)/(time been in space) or instead you could do years since the beginning of the universe minus how long it took multi-cellular organisms to come into existence since earth was made.