There are no hard and fast rules about gain reduction when using brickwall limiters like Waves Audio L2 Ultramaximizer or Waves Audio L1 Ultramaximizer. It's all about the desired sound you can achieve by using any audio mastering limiter. Although in contemporary times pushing Waves Audio L2 sounds nasty compared to other modern VST plug-ins.
Perhaps a guideline is somewhere around -1 dB to about -3 dB of gain reduction with the Waves Audio L2 plugin and then proceed to use a second VST brickwall limiter. It's all a matter of taste. For me, I personally wouldn't use Waves Audio L2 as a final brickwall limiter because both FabFilter Software Instruments' Pro-L and iZotope Ozone Maximizer sound way better, to say the least.