Basically you compare your final gravity and the amount of grains in lbs to what the max pppg is for a given grain summed per grain, per amount of your total grain bill. The other thing is that grains aren't always exactly what the numbers are cited at, but for the most part they are fairly consistent from what I understand. This is your extraction efficiency I believe. Oh plus the OG is pre-boil and you need to accurately measure the amount of wort collected.
Event Horizon ~ A tribute to the miracle of fermentation.
Brew what you like. Do this, and you will find your inner brewer.