Well a few things that I got from talking w/ the guys @ white Labs (those guys ROCK!) is you want to keep your starters around 1.038 or so. You are trying to get the yeast to the reporduction stage not the alcohol producing stage. So, yes an OG reading will help. 800 ml is an OK place to start but I look for 1200-1500 Ml @ 1.038 and on the plate I will see full Krausen in 18 hours or less.
Giving it a shake is a great idea or I tend to tell people a "swirl" every time I look at it swirl it if you dont have a plate. without a plate you may take a little longer to hit high Krausen.
What strain? Straight from the vial or a washed used sample?
Cheers
Jay