Originally Posted by tootal
Definately your boil rate. The design and size of your kettle and the amount of heat all can factor how much you boil off. You want your boil to be a rolling boil. In other words as big a boil as you can make it without boiling over. I never concern myself with the pre boil gravity. We always do a 90 minute boil at the same rate of boil, starting from the same volume in the kettle. When all things are equal we nail our OG every time.
I think this really nails it on the head.
Now that I have done a few batches I have all my numbers dial in pretty well. I tracked my pre-boil volume, gravity post boil volume and gravity and after a few time I was able to set the efficiency in beer smith so I hit my numbers nearly every time now (+/- .01).