11.234 # per gal X 10 gal X .7 = 78.6 # of extract you're adding
78.6 lbs / 7.5 bbl = 10.5 lbs. extract per bbl
Using my handy Siebel Brew Computer (everyone should own one of these, they're only a few bucks),
12 deg. P wort has 32.4 lbs. extract per bbl.
if you add 10.5 lbs. extract per bbl,
42.9 lbs/bbl equals about 15.65 deg. P.
So in this example, it added 3.65 deg. P.
I didn't account for the volume increase (almost a half bbl) of the 10 gallons, but you get the idea.