It is really a matter of personal preference. 'Backsweetening ( if that's a word) to taste' is exactly that, sweeten it to what you prefer. You can add concentrate, apple juice, corn sugar, etc. You said you were going to keg, so it is up to you. Somewhere between 1-3 cans of concentrate, or 32-64 oz of juice, or a pound of sugar, into the keg, seal it up, cool it down and put the CO2 on it.
I'd say add some form of sugar until you get a taste you like, then seal the keg and keep cold.
If you did NOT do something to stop the yeast ( Camden tablets), then the yeast will eat thru whatever sugar you add to sweeten if kept at room temp. If you put the keg in a cooler ( 40ish degrees ), you will be ok for months, but if you leave the keg at room temp, the back sweetened sugar will just turn into alcohol.
This is a start, thank you. I don't plan on placing it in the kegerator right away because it's only 3 months old (I need the carboy), so I guess I'm going to have to look into the camden tablets before I back sweeten.
George