1. Most of it has to do with the subject's hair color.
2. What YOU expect out of a hair light.
A subject with blonde hair, will require less exposure (on the hair) than a subject with black hair. If you're shooting digitally, you should be able to adjust on the spot. However, if your shooting film then you're just going to have to test subjects with different hair colors to get the effect you're looking for.
Here's some starting suggestions (based upon your taking aperture).
Black hair: 1 to 1 1/2 stops over
Brown hair: Same as your taking exposure
Dark blond: 1 stop less
Blond: 1 1/2 stop less
Very light: 2 stops less
The above are only (my)suggestions to start from a point of reference. Everyone will have different opinions of what they want to "see" with a hair light.