The expected OG for extract, unlike All Grain, is pretty much written in stone for a given batch size. LME typically adds 36 gravity points per pound per gallon. DME adds about 40 gravity points/lb/gal.
Promash makes it really easy to do the calculations but it is possible to do it with some simple formulas. For example, the formula for your 6 lbs of LME would be 6 lbs x 36 gravity points ÷ 6 gallons = 36 or .036
Then the 2.5 lbs of DME would be expressed as 2.5 lbs x 40 gravity points ÷ 6 gallons = 16 or .016
Add the .036 to 0.16 along with the given of 1 and you get a SG of 1.052 for your 6 gallon batch. Change the batch size to 5 gallons and plug that into the above formulas and you come up with a SG of 1.063.