Do like Steve suggests.
I make each year elderberry wine.
Now This year I made 2 batches, each using the same recipe.
Both batches 30 liter.
For both batches I added enough sugar to get to 15%
alcohol. But that is not what I aim for.
I aim for a 12% - 13% wine with a lot residual sugar.
The first batch did as I wanted. Indeed 12% and a
lot residual sugar. Right on the nose. A lovely immediately
drinkable dessert wine. Great for Xmas.
The second batch did his own thing.
It fermented dry.
It also had a lot of tannins, so now it has to age
for several years to become drinkable.
So as I wrote on my web-log:
http://wijnmaker.blogspot.com/2008/12/lange-termijn-experiment-in-long-run.html
I made a long term project with it.
So shortening things down.
It is unpredictable what wine yeast will do.
It is possible that they ferment an overly sweet must
to dry, or they quit at a lower alcohol percentage and
you will have a sweet wine.
So aim at the alcohol percentage you want, let it ferment dry,
and then stabilise before adding sugar to sweeten.
Luc