Paul, it’s a 3 gallon kit.
I was looking at the sg readings and I calculated the alcohol as 16.25%. This is what I did.
Initial sg was. 1.120. Fermented down to 1.017
1.120 minus 1.017 is o.103 times 131 is 13.49%
added chaptalization pack sg was 1.028 fermented to final sg of 1.007
1.028 minus 1.007 is 0.021 times 131 is 2.75%
I added 13.49% and 2.75% is 16.24%
Does that make any sense ? Or am I calculating it wrong ?
Okay, this is now enough information.
Firstly, your calculation is not technically correct, but it is close. This is the SAME calculation that I offered in post #4 as the "wrong calculation." Technically, it is not correct because it ignores the increase in volume upon chaptalization. But, as the back-and-forth with @BernardSmith
pointed out, it is a close approximation.
But now you have given me enough info to back out a close approximation of the added volume! I will below perform the "correct" calculation, just for the record. It won't be much different from your estimate, I am sure. Leaning on Fermcalc...
First, how much alcohol did your first fermentation produce? You went from 1.120 to 1.017. As you say, our usual calculation gives us 13.49% ABV. [I will use this figure, but note that Fermcalc gives other values (depending on the calculation methods) ranging from 13.9 to 14.4. For simplicity's sake, I will use 13.49.] But how many ounces of ethanol is that? For 3 gallons (384 fl. oz.), it is 0.1349*384 fl. oz. = 51.8 fl oz of ethanol. We will need this later.
You added enough sugarwater to increase the SG from 1.017 to 1.028. Fermcalc tells me that if I add 340 g of sugar to 3 gallons, I get 3.0554 gallons of liquid with a SG of 1.028. But your pack weighed 400 g., so there must have been ~60 ml of water. To an excellent approximation, your volume after chaptalizing was 3.0554 gallons + 60 ml = 3.071 gallons = 393.12 fl. oz.
How much alcohol did your second fermentation produce? Your must went from 1.028 to 1.007, increasing the ABV by (1.028-1.007)*131=2.75% (as you calculated). This means that your post-chaptalization fermentation produced 0.0275*393.12 fl oz = 10.8 fl oz of additional ethanol.
So, all told, you have 51.8 + 10.8 = 62.6 fl. oz. of ethanol. This composes part of 393.1 fl oz total liquid volume, so the ABV is 62.6/393.1 = 15.9%
Again, as I said in post #4, the simple method yields a close approximation, but is a slight overestimate (by ~<0.5%). Note that the different methods of calculating ABV (Duncan, Berry & Acton, etc.) produce variations as large as this difference, so my mentioning this dilution effect was probably unnecessary.