Unfortunately the Extrude % field doesn't work for setting a depth with the last 6 brush types in the "E" panel.
Your other tip about using the coordinates is great though, thank you.
What I want to do is be able to trace a background reference image to create my initial volumes. It's already possible with the grow tool and one of the last 6 brushes in the "E" panel, but without a usable depth setting.
This seems like the absolute easiest way to start a hardsurface model, why mess around with primitives that are not the exact shape you want when you can just draw a volume that is the right shape?
So far my best workaround is drawing the shape, cloneing it, stacking them as many times as I need, and merging.
Still hoping that someone knows the answer to this riddle, it seems inconceivable to me that there's no way to set a depth with those brushes.
Thanks