As noted, the AUD to SBD ratio is going to be dependant on a lot of external factors, such as whether some/all of the instruments are miked, how good the room PA sounds in general, how hot things like the kick drum and other instruments are in the SBD, how "chatty" the room was with reference to where your mics were, and many other things. It really comes down to what you think sounds best. Some recordings only need a little SBD to bring up vocal clarity, and others need a lot more help, if the room sound or mic location were not optimal.
You may want to consider some compression on the SBD, particularly if the kick drum (and/or keys) are way hotter than the vocals. Otherwise, you will be stuck with a potentially overbearing kick when you are just trying to get some vocal reinforcement.