10 points off is pretty significant. How much wort are you losing due to BK deadspace/hoses/chiller?
Let's say you just finished your mash and you have 7 gallons of 155° wort pre-boil at an SG of 1.045. Water expands about 2% at 155°, so you actually have about 6.85 gallons of wort.
6.85*45 = 308 GU's.
And you do your calculations and say, "well, if I have 308 GU's and I want 5.5 gallons of finished wort...lessee here...308 divided by 5.5 equals 1.056, and HEY, that's my target OG, so I'm golden!"
Then you do a boil for an hour and you evaporate 1 gallon of water and you stop the boil with exactly 6 gallons at boiling temps.
You're going to lose about 4% of the volume when the wort cools, so you have about 5.75 gallons of cooled wort. And let's say you have a very efficient system and lose only .25 gallons of wort to BK deadspace/hoses/chiller, so when all's said and done you have 5.5 gallons of wort in your fermenter just like you wanted. Life is good.
So you go and take an OG of the wort in your carboy and...1.053?! WHAT THE HECK?! WHERE DID THAT THREE POINTS GO?!
But think about that quarter gallon you left behind and the GU's it contained.
308 / 5.75 = 1.053, not 1.056. MATH DECEIVED YOU AGAIN. CURSE YOU, MATH!
So how do you compensate for this problem? By figuring out how many GU's you're going to lose from your BK deadspace/hoses/chiller and adjust accordingly.
On my system, I lose about .75 gallons of wort to BK deadspace/hoses/chiller. I have a 25 gallon pot, so yeah, the deadspace is just going to be bigger.
Anyway...so let's say I have a pre-boil SG of 1.045 at 14.5 gallons (measured), and I know that I'm going to lose .75 gallons of my volume down the drain.
.75 * 45 = 34 GU's that are going to be lost to my system, so I need to deduct that from my GU count.
14.5 measured gallons minus 2% volume for (155°) shrinkage is about 14.2 actual gallons.
14.2 * 45 = 639 GU's.
Minus my 34 GU losses to my system, and I'm looking at 605 GU's that will be going into my carboys.
I'd like 11 gallons of finished wort, so...
605 / 11 = 1.055.
Is that what I'm looking for? If it's high, then I add more water or don't boil as long or I just deal with the fact that my mash tun is trying to get me drunk. If it's low, then I boil longer or add some DME to up the GU's or just deal with a lower OG. If it's spot on then I celebrate. "HA HA! I HAVE DEFEATED YOU ONCE AGAIN, DEMON OF INCORRECT PRE-BOIL SG!"
Anyway...this is my process. It seems to be working pretty good for me so far, but others may have problems with my dumb methodology.