There is a range where anything between the max(cutoff) and minimum bias(plate bbq) voltage is OK, then you choose somewhere in between that range according to your needs. Center would be somewhere in the middle, I guess.
How do you find the approximate middle? For guitar amps, techs normally shoot for 70% of full tube dissipation. So, if you have +B voltage and you know the tube's wattage, bob's your uncle: you know the 100% current. Take 70% from that and you've got the current to shoot for.
This normally isn't done for 12AU7 tubes though, this is power tube stuff. For preamp tubes their maximum dissipation is 1 to 3 watts max, so you're not really worried about quiescent current, but about tone shaping, frequency response, etc, instead.
Good luck with the project.