Calculating ABV when adding sugar during fermentation

[White_Water]

Hi newby here. It's my first attempt at brewing a big beer so used the following recipe:

http://beerrecipes.org/showrecipe.php?recipeid=1215

I used slightly less Maris Oter (due to package sizes I would have wasted 90% of a .5kg bag) and ended up with 25.5l of wort 1.057 pre boil. After boil and transfer to fermentor I ended up with 21l at 1.073. (btw I expected the low efficiency as my mash tun is not very big but I thought I'd try anyway).

All the above is without the 8oz of Turbinado (will use Demerera instead) which after reading a few threads I have decided to add in 2 batches of 4oz boiled in 1l of water (or more if its too thick).

I have tried to find some info on how to calculate the ABV for the above but am a bit unsure if I am calculating this correctly so would appreciate some feedback. If I follow the calculations here to estimate the gravity of my additions:

https://www.homebrewtalk.com/f13/add-sugar-after-fermentation-starts-calculate-abv-422727/

Target O.G. = (original volume x O.G. + additional volume 1 x addition 1 O.G. + additional volume 2 x addition 2 O.G.) / (original volume + additional volume 1 + additional volume 2)

85 = (21x73 + 1xX + 1xX) / (21 + 1 + 1)
85 = (1533 + 2X) / 23
X = 211

So that's an O.G. of ... 1.211 which looks a bit high

So the question is, have I made a mistake in my calculations or am I just aiming too high and will not be able to reach my target O.G.? Not a big deal but would just like to know how to calculate the ABV with some accuracy.

 

[ong]

Sugar adds .046 points per pound per gallon, so about .009 points per 5 gallon batch. Half a lb of sugar would get you .004 points or so.

(You don't need a full liter of water for just half a on., BTW -- I'd just do a cup or so, as needed.)