Bench Trials - Take a cup (8 oz) make a simple syrup if you are going to use cane sugar to sweeten. (1 Cup hot water 2 Cups Sugar)
Sweeten with your simple syrup using no more that about 1/8 oz at a time. Note how much got you to just about what you like (perhaps a little dryer than perfect)
Note how much you added then - for a one gallon batch use 15 times that amount of simple syrup. (16 cups per gallon but you already sweetened one cup) Most of the time I round my number down to the next lower 1.2 oz of sweetener just to be safe. So if I needed 3.75 ozs for my gallon batch I drop that to 3.5 ozs - Since you can't un-sweeten it.
After I have sweetend the batch then I take the SG reading for my label and record keeping.
BUT BEFORE ALL THAT you should have added your K-meta and Potassium Sorbate - about a 3 days to a week before you sweeten.
I also like to let it set another week after back-sweetening just in it develops fall out or a haze. (Have a peach that got slightly hazy after sweetening but 4 weeks later it has cleared up nicely. Ready to bottle sometime in the next week.)
Farming looks mighty easy when your plow is a pencil and you're a thousand miles from the corn field.
(Dwight D. Eisenhower)