Did a quick search, but either nothing specific came up, or too specific (math I am too tired to try to comprehend right now). So my question is this: how do you adjust gravity when combining volumes of different gravity?

Here's my example:

I am making one gallon batches of cider (upstate mikes Carmel apple, in this case, with some adjustments). I took a reading of the Kirkland (Costco) juice I used;

Was 1.047 (at 79 degrees - but so were the rest of my readings, so shouldn't have to convert based on temp, right?). I poured half of the gallon into my 1gal glass jug, and added 1/2 cup dextrose to the remaining 1/2 gal in the plastic jug. After shaking/mixing it in, my reading was 1.062. (I also did a second batch using 1 cup dextrose - for comparison - and got 1.079 in the 1/2 gal of that one.)

I know that to convert to gravity points, you multiply volume by gravity. So I'd have 0.5 gal X 62 = 31 gravity points (or 0.5 X 79 = 39.5 grav pts for the second batch), and have 0.5 gal X 47 = 23.5 for the plain juice. So do I just add the two together (because they are equal volumes added together) to get the gravity of the combined one gal mixture? Thus... 31 + 23.5 = 54.5, or 1.055 (rounded up) for the first batch, and 39.5 + 23.5 = 63, or 1.063 for the second?

It seems intuitively correct (setting aside the fact that the sugar dissolved in the juice may increase the volume above 0.5 gal a bit... But should be negligible, no?), but just not sure if I'm doing the math correctly. I just want to make sure I'm then calculating the abv correctly (am factoring temp diffs/corrections to readings here, of course).

Thanks for any input!

(FG was 1.003 for first and 1.000 for second (corrected to 60*), after 12 days, having pitched onto a Nottingham yeast cake from two previous one gal batches (which is why I didn't take the OG reading from the mixed gallon after, I guess? Didn't want to lose the volume of the sample, or risk oxidation by putting it back into the jug with the yeast... I guess? I don't honestly remember!) Anyway, should give me an abv of around 7.2% for the first and 8.6% for the second, if my math is correct...)