Pre-boil SG and OG are different, you want to measure your gravity before the boil and after it goes into the fermenter to calculate your brewhouse efficiency.
If you have a graduated bucket, keep track of your firt runnings, and sparge runnings (this will be your total pre-boil volume and when to measure yor pre-boil SG), normally you'll boil off about a gallon or so in an hour long boil depending on your equipment. If after your 60 minute boil you have less than 5.25-5.5 gallons then you will probably want to add some top-off water to achieve the proper volume (a graduated bucket makes this relatively simple). After adding to get your desired volume make sure your dilution water is mixed well and take your OG then pitch your yeast. Normally, if you boil off more than expected and have to add some water back your gravity will be very close to where it should be. In essence boiling is concentrating the wort, so adding water back is diluting it.
Also
FWIW, my first AG pre-boil volume was 7.5 gallons, after the boil I ended up with about 4.75 gallons, so I added about .75 gallons back in to achieve a volume of 5.5, my estimated OG was 1.045, I ended up with OG 1.046. I will adjust my pre-boil volume to be 8-8.25 gallons.
