The best way to know is to keep a log and figure out what YOUR boil off rate is on average, so that you can know ahead of time how to adjust for future batches. Everyone will have different rates based on the size/diameter of their pots, intensity of their boils, and other factors like elevation, etc. Again, best way to figure it out is to keep good notes. Also, you can always top off with water if you under shoot, and boil longer if you over shoot (but that can mess up the hops schedule if you're not paying attention). So, in my humble opinion, it seems better to undershoot until you have your own data to work with.