Doober...
If your starting SG was 1.080 then
your Brix = 20 (at start)and
your abv = 11.5%
Your Brix now = 0.0 but will of course, increase when you back sweeten.
Regarding Backsweetening.
Wade is 100% right!!!
Go slow on your sugar additions, maybe 1 cup at a time and each time you add the cup, stop and taste it (after mixing well) and measure it.
By measuring, you get an idea of what degree of sweetness you want to shoot for in your end product next time.
If you're unsure now, take a thief of your favorite store bought wine in your hydrometer flask, measure that sg. and pour the contents back into your wine glass. Then, while you sit noting the results, enjoy that glass of wine.
When you add the sugar be aware that your must's volume will risein equal proportion to the amount of sugar (and water)you add so be sure youhave roomfor it.
I like to do my back sweetening after I rack because of the natural lessening of volume that occures due to siphoning.
Another thing to consider here is, if you add water you'll be diluting the fullness of your wines bodyto a certain degree. In some cases that may be beneficial, like if it's currently too bitter or tart.
What I usually do, is after racking I remove a cup or twoof must and add my cup of sugar to that, mix well and return it to the carboy. This keeps my wine'sbody moreconsistant,I'm not fighting the negetive sg.effects of the water additions and the volume doesn't rise any more than the volume of the sugar.
Your wine looks great!
Bear in mind also, that time and patiencewill clear a wine in most cases as well as your additions of clearing agents will.
What's next on your project list?