I always bench test with small samples. Get the samples to the
temperature you will be serving at then add small amount of sugar and
taste then add a little more if necessary and taste, etc. until you
find the level you like. Then extrapolate that amount to the total
amount of wine and sweeten it all.

If you get a few people to help with the taste testing you can blame
someone else if it doesn't turn out right in the end.