ZBrushCentral

Setting maximum depth

I am completely new to zbrush so I’m not sure if this forum is the right place to ask what is probably a very basic question. Apologies in advance if it isn’t the right place. Anyway, I want to try to create a model that could be ‘engraved’ on a coin or medallion. How do I set the maximum ‘depth’ (e.g. to 0.18mm)? Do I set the parameters before I start modelling or do I scale the model afterwards? Any help gratefully received.
Thanks