@ the OP - I have been looking at your busted bit and I think you should check your setup rather than blaming over tightening.
I have found that certain combinations of saddle, saddle angle and seat post angle do not play well together with the retro USE posts ...
I suspect if they don't work well together it will rotate that clamp into a position where the edge of it takes the stress of the riders weight and that could lead to the deformation you can see on yours. What is the wear pattern like on the underside ? Is it central ? I suspect it will be offset towards the damaged edge.
For info, a USE seat on a retro USE post won't work on my Airborne ... :-(
WD