I am assuming that by "boil gravity" you mean the gravity of the wort before the boil. The way I would solve for that if I knew I wanted 10 gallons of boiled wort that was 1.060 would be to multiply 10 times 60 which equals 600. If you are going to start with 13 gallons, (and boil away three) the gravity at the beginning of the boil would be 600 divided by 13, which is about 46, or 1.046.
Pre-boil. Though it's more complicated than that. Because you're evaporating and concentrating the wort, the utilization changes over the course of the boil. You may start at 30% and drop to 25%; which skews your IBU calculations.
Speaking of which, do you know your kettle utilization?