Sorry I didn't understand you question.
I'd yank out the algebra and figure it out by the numbers. I've found that the calculated and measured are close but not the same
So if you add a lb of dme then add 45/brew size to the og.
But that is just me
Right so where the algebra comes though. If your brewday OG and volume are 1.060 and 5 gall, then you have 300 points (5 *60) and if you ad .5 gal and 2lb of dme that is 90 more points and .5 gal vol - so now 390/5.5 gal = a calculated og of about 1.071 ... then latter you add another .5 gal and 90 points of dme for 480 and 6 gal or 80 points and 1.080 calc og
Again, that is how I would figure it. If I were adding a small amount of sugars - say 1/4lb and .5 gall, I'd probably figure that my gravity ahsn't changed much, although someone could do the math and show me I'm wrong