How do I determine the og after the addition of more fermentables

My first ag batch was right after a cool front came through and I boiled off 2 gallons in 60 mintues. So, that being the first time i've used that whole system I figured i'd go with that.
Turns out it was a lot more humid for my second batch and I started with 8 gal preboil and wanted 6 post. I'm not sure exactly how much I got but I filled the normal "brew pail"
all the way to the rim and still had some left in the kettle. Anyway I think I have 5 gal or so of 1.060 wort ( supposed to be 1.072 ) in the brew pail and I'm going to just leave
that one alone to do it's thing exactly like the recipe.

The other ~2 gallons I have in another fermenter and I was thinking of feeding it some extra fermentables. Dextrose or turbinado or something just to see what it ends up as.
I'm really not concerned with screwing it up.
My question is: If I have 2 gallons of 1.060 wort and 1lb. turbinado has an sg of 1.042 ( according to beersmith ) whats the formula to figure out the new og?
2 gallons 1.060 + 1/4lb. turbinado = ?

2 gallons * 60 points per gallon = 120 points of sugar in your beer
1/4 lb * 42 points per lb = 10.5 points in the sugar

120 points + 10.5 points = 130.5 points total sugar

130.5 points total sugar / 2 gallons = 65.25 points per gallon = 1.065 new OG

That is correct. If that is confusing, try this:

Figure out the total gravity points you are adding. 1.042 = 42 points. So 2 lbs would be 84 points.

Divide the total additional points by the number of gallons. Say 1 lb in 2 gallons = 42/2 = 21. Then add this number to the original starting gravity; if 1.060, it would = 1.081.

nevermind