For back sweetening, I take a gravity reading from the batch "as is", then I make up "honey syrup" (50/50 honey water). It then depends on what the gravity reading was. I'd say something like 50mls per gallon to start with. After the first addition, take another gravity reading (don't forget to stir the batch gently after you've added the syrup and before you take the second reading).
The second reading will tell you how much you've increased the gravity by, and taking a little taste of the sample will give you some idea of whether you need to add more.
The trick is "a little at a time".
I like my meads at about 1.010, which most would consider medium, anything above 1.020 is too sweet for my liking.
regards
fatbloke
p.s. Oh and as Yooper says, stabilise first. You can add the campden/sulphite and sorbate at the same time. I then like to give it a couple of days before I start back sweetening.