if you're using photofloods, you can work down using a scrim (metal window screen material) on a c-stand w/arm. Dimmers can work too, but I find it better to control the right amount of light by layering scrims. As far as contrast ratio are you following the K+F /F (F= no key) formula? If so, my general rule of thumb for blondes/white hair in front of a solid (darker color bg) is you want your hair light to match the key. Marlyn Monroe blonde = match key; strawberry blonde = a little more playing room, but never by more than a full stop over (or one under on the high-end of my workable latitude) unless I'm going for an Orwellian Citizen Kane look, say an 8:1: I'd like my hairlight to be be 1 1/3-2/3 over key.
forgot to mention: contrast ratio in exponent form eg: 2^2 = 4 (2 stops)