There are no rules Just go slowly as if you over sweeten you will have to dilute it with another wine to bring it back to dryer. I usually sweeten back a little under where I will want it cause the sweetness will come through more with time just as the fruity flavor will to. I do the opposite with oak as that seems to fade a little so i actually over oak it just a little bit.
I use my Hydrometer to test a sample and go by the chart on page 15 of the book George gives away called "Enjoy Home Winemaking" but its pretty simple.
Dry Wine .99 - 1.00
Medium Dry1.00 - 1.008
Sweet over 1.008