Ok, that SG reading is the reading of the wort after the mash and sparge, right? Total pre-boil volume? Maybe a dumb question....
No, I think you mis-understand.
The SG of your wort after sparging is called the "Pre-Boil Gravity". This is the reading of all of the wort in your boil kettle before the boil starts. It is usually 15 - 20 points or more different (lower) than what you expect to reach when the boil is over and moisture and other volatiles (DMS) are boiled off.
The "run-off, specific gravity" is the reading of the liquid that is actually draining from your mash tun, as it is draining. So, you would take a cup full of the liquid running off the mash and measure its gravity before it hits and mixes in with what is already in the boil kettle. If it reads 1.008 or lower you need to stop sparging, no matter how much liquid is in the boil kettle. In earlier stages of sparging this reading will be much higher.
If you have your water volumes and efficiencies dialed in for the amount of grain you are using in your recipe, you really don't have to worry about this. Recipe programs, like BeerSmith, calculate this for you. Your final runnings should never get that low unless you aren't using a recipe program and are flying by the seat of your pants and just going with the flow, so to speak.
Not to muddy this up, but there is a type of brewing called "Parti-Gyle" that takes advantage of the second runnings of a mash used for a real high gravity beer, like an Imperial Stout or Barley-wine. These beers use a lot of grain to extract a lot of sugars. Since you want to keep the SG higher for these high alcohol beers, you don't sparge until all the sugars are rinsed and you leave some behind. This is because you don't want to dilute the high sugar wort down much. If you did you would have to boil quite a while to boil off enough moisture to reach the gravity you need to obtain the high alcohol inherent in these beers.
With a Parti-Gyle, at the point where you have the volume you need for boiling the high gravity beer, the runnings coming from the mash tun may still measure as high as 1.040-1.045. Not wanting to waste this good wort sugar, many brewers will sparge into another container until the gravity from these "Second Runnings" reaches 1.010 or so. They then do a completely separate second boil and create another beer from the same grain bill (or maybe steep or separately mash some additional grains to really make it different). They may be able to get a few more gallons of this lower gravity wort which would make a great session beer. Different hops schedules, yeasts and other changes can make a beer you would never guess was from the same grain bill.