Stage lip recordings capture the sound coming off the stage which is NOT coming from the PA system.
A loud band in a small club might only have vocal mics running through the PA speakers. A SBD patch from that system would only give vocals, so you'll want your mic pair to capture everything else.
Stage monitors aimed at the band provide the vocals they need in order to hear each other, but that sound doesn't cover the audience.
Knowing you'll be getting good vocals (for instance) from the SBD feed, you now have the option to position your mics "UNDER" the stage monitors to keep that vocal (mostly) out of your mic feed!
I have found it easier to mix stage lip recordings with a symmetrical setup, whether closely spaced, or split.
My personal rule of thumb is that the kick drum is probably going to be one of the loudest sounds on the stage, so I endeavor to make sure my stage lip mics are evenly spaced from the kick drum. If the kick drum is off center, I'd probably aim at the center, but the kick drum gives me a bit of a target since it's often right in the middle.
As jefflester points out, anything NOT loud on stage will probably be added to the PA mix in a sort of inverse proportion. This typically includes acoustic guitars, electronic keyboards, electronic drums, sample playback, and any sound effects added to the vocals. So that's what will most likely comprise your SBD feed. Position the mics to get the "BLAST" and feel of the band. Depending on capsule type, and stage layout, you might get better results on the corners, in the center, or midway. Other things to consider with stage lip recording are fans placing drinks, and guitarists clicking pedal boards or kicking mics over!?