There is also some very good reading at the yeastcalc site. http://yeastcalc.com/
I use this site for my starters and only this site, as it seems to be spot on and up to date.
Using consistent pitch rates is important to getting consistent results. I have tried the same recipe with the same yeast at different pitch rates...you would swear they were two different beers. So, get one of those syringes with the "ml" reading from your local pharmacy.
I have read there could be anywhere from 1 to 5 billion yeast cells per ml of slurry. I use the figure of "2 billion per ml"...and it seems to work out nicely.
Now that you are washing, move on to freezing your yeasties too. The freeze dried kind are the best to play with since they are cheaper than the premium strains. I freeze my yeast in these 8 dram bottles from Amazon
They hold 29 ml of fluid. I fill mine with 14 ml yeast(I figure 28 billion cells) slurry before I freeze. I personally figure 33% survive the freeze/thaw process, giving me 9.3 billion healthy yeast to put in my starter. More than enough, as I have read of some people that do slanting, only freeze a few MILLION cells, and still get good results.