I wondered if anyone had advice on the following matter...

I am making a rice-wine and I'm using gervin gv26 yeast. It states the capability to reach ~21% vol and my intention is
to get as close to that percentage as I safely can. I want the wine to ferment out dry, so while this is an experiment,
I don't want to push it too far and find myself high and (not) dry.

So far, the 5UKGallon wine has had 10.45KG (23 lbs) of sugar which would seem to put it somewhere north of 18.5%
alcohol - assuming it ferments to sg 1.000. Today, for the first time so far, the fermentation seems to have slowed
slightly.

I have been tracking the SG drops, but with some anomalies (for example addition of 3 litres of water some time after
starting didn't seem to change the SG - perhaps due to post addition aggitation adding straining bag contents back into
the liquid). Annoyingly I didn't take a pre-sugar SG. If the SGs were all believable the wine would already have made
a 0.182 drop making the wine ~25.5% which is clearly erroneous)

The question is when to stop feeding the wine with more sugar. I don't want to push it till it will ferment no further
as I will end up with residual sugar. I am happy to stop where I am if it is the only way to achieve my 'dry' goal.
The ~5UKGallon batch had 55g of minavit nutrient to work with - a compromise on the amount stated to achieve full
potential.

Does anyone have any experience of this kind of scenario or vaguely similar which I can use to extrapolate likely
success with further sugar additions?

Many thanks for any advice,

Jim