Unable to thicken a face by a small amount

As a fun project I’m trying to model the house I live in. The walls are made of brick with a plaster covering. Initially I applied a separate material to the external faces to mimic the situation, but ultimately decided to duplicate the faces then thicken them before applying a bump map.

(If anyone is interested why I’ve gone this direction, it’s because the approach mirrors the physical situation as well as allowing me to use clipping planes or layers to reveal the building construction.)

I’d like to set the thickness of the plaster to 5mm. The model units are metres, with an absolute tolerance of 0.001 units (i.e. mm) and display precision also of 0.001.

Unfortunately (and it may be me still figuring things out) when I go the thickening setting, it only allows a precision of 0.01. This leaves me with 1cm plaster which is causing some minor – but irritating – inconveniences.

Is this a bug, or is it my doing that I’m being bugged by it?