It's based on a quick n dirty unverified lab test that I did. My value before priming was similar to the value from the hydrometer tests. I also tested a commercial microbrew and found that my value was about 0.5% higher than on the bottle, for what that's worth.
I also wondered this, so I calculated it for dextrose, using the molecular weights of dextrose & ethanol, density of ethanol, and reaction equation (1 dextrose --> 2 ethanol + 2 CO2)
A simplified formula for this is:
Additional alc content = 0.48 x (oz of dextrose added) / (US gal of beer)
So...
To add a little nerdity to this...
Assuming dextrose was used, and was completely converted...
Every 1 molecule is converted to 2 molecules of ethanol and 2 molecules of CO2.
4 oz of priming sugar into 5 US gallons of beer will increase the alcohol content by a maximum of 0.39%v/v.