I try to avoid backsweetening, but if you do a forum search on "backsweetening" you will find more posts than you can shake a stick at.
Before adding sorbate, I strongly recommend you taste a little bit of it first. It leaves a very distinctive taste, which some people are OK with, but most avoid. If you are OK with the taste it is very effective. If you dont like it, better to toss a 4oz glass than have to drink 5 gallons of it