Not quite sure what the question is here. You can lower the however much you want? You'll be limited more by ofsets & width than rim size. You'll sit slightly lower on 18's when slammed if thats what your asking.
it may be better if i ask this... how low can i go without having any issues with rubbing etc with (19'' et45 8inch front, 8.5 rear) does that help? tyres either 215/35 225/35