It depends on if I hit my preboil gravity.
If I did, then I change nothing, and make extra beer.
If my preboil gravity is too low, I extend the boil off rate to reach my anticipated OG. I have slight variations in boil off rate, so I have a range I'm shooting for. So if its just a little off, I let it ride.
If my preboil gravity is too high, I can add water to to adjust, then do my normal boil. And make extra beer.
So basically, I use preboil gravity, and anticipated OG to determine what I do with my boil.