Thanks Revvy. I had my first boil over today and I had used Fermcap. I put it in right away. I have a 7.5 gallon kettle and I started out with about 6 gallons of water, anticipating boil off.
I wait until I see the foam start to climb towards a boil over, then I add it a drop at a time until it starts to fall. I usually have to do that twice and after that it doesn't climb again.