Sorry Phil I can't help you with that one not using the Spyder myself. I use the EyeOne2 and at the beginning of the calibration process one has the option of having the device complete an auto calibration or you have the option of setting things manually. I don't opt for the auto function because it uses the monitors native white point which I find far too bright. I'd check the instructions once again and/or the support site for the Spyder because I feel sure that there must be an option of the user setting both the luminance and gamma manually.
Edit: I've just checked colourvisions site for myself and their is a pdf file there showing the screen setup for the Spyder 3 - presumeably the latest model. Yes, it would appear that from the beginning of the calibration process you are able to enter values manually. You want a white point of 6500K, Luminance of 120 and a gamma of 2.2 (1.8 if on a Mac).