What I should have been looking at is the Geoid of Mars & the depth from it to the mantle
The thickness of the crust varies between 50 km & 22 km due to the Geography & features (rock layers) above the Geoid while the distance from it to the mantle should be reasonably uniform.
I presume the putative Geoid of Mars is the altitude used for measuring atmospheric density.
According to some sources the crust of Mars is 10 km thick at its thinnest points which is presumably to be found in those places furthest below the Geoid like the Hellas Planitia . which means the mantle is around 17 km below the Geoid & may mean we can't go much deeper than 7 km without lava flows.
So the crust beneath the Hellas Planitia is probably only 10 km thick.
I found this KSP compatible Mars Heightmap
& this map of the topography of Mars (useful as you can zoom in & out on features)
The Interactive Mars map at the bottom of WikipediA's 'Atmosphere of Mars' page is also useful as it names features you hover the cursor on & links you directly to their page if you click on them.
I find the fact there appear to be no features with a bottom closer than 10 km from the mantle suggestive that this may be the hard limit .. so, anywhere the crust is 50 km thick we should be able to dig a hole as much as 40 km deep if we want.
It also means we probably can't achieve a depth below the standard atmospheric pressure of Mars of much more than a few kilometers beyond 7 km without being in danger of getting magma .. the 0.168 PSI of Hellas Planitia (air pressure at the summit of Mount Everest (the highest point on Earth) is 4.89 PSI) is probably not that far from as good as we can get on Mars by digging holes .. not what I hoped.
The peak (presumably a post impact volcano) at the centre of at least one crater close to this depth (Lyot) seems to support the idea a point on the crust thinner than 10 km is likely to pop.