Boiling causes evaportation which condenses sugars in your wort. Your starting gravity reading is based off of the post-boil/pre-fermenting gravity. Odds are if you boiled off more than you were supposed to (boiling too vigorously, wider pot, etc) your starting gravity will be higher due to the more concentrater sugar content.
If your OG was supposed to be 1.05 and it ends up being 1.065 for example, you could simply add more water and give a good stir before transfering to the carboy/bucket. Ultiamately you would only add enough water to bring your gravity down to the recommended reading or until you hit desired volume, whichever comes first.
Get a stick or piece of long and skinny pvc pipe or something. Put gallon of water in the boil kettle and mark the stick/pipe with permanent marker or a notch. Repeat over and over until you get about 7 gallons marked. Between gallons 4 and 7 mark half gallon increments. As you boil drop the stick in and it should tell you what your volume is. Sample your wort 15 mins before the end of the boil and cool it, check gravity. If you are a half gallon short, gravity is probably gonna be over target. If so add water now and keep finish the boil. THis way you don't rack to primary just to find out you have way too little volume.