I think.

I made that calculation ages ago, maybe it was 9%. Maybe it was 15%.

But it's somewhere generally around 10%. I think.)

18.6% flat structure

18.6% flat structure

21.7% flat structure

27.9% flat structure

(ROI means return on investment, just an arbitrary value where higher is better)(flat structure is what the bonus actually says. "Structure value" is 90% of that, because most weapons on average deal about ~~10% extra damage to structure. 25 points (your version)14.4% armour (0.576 ROI)16.7% structure value (0.670 ROI)---> 1.2456 total ROI 27 points (extra points)16.0% armour (0.593 ROI)16.7% structure value (0.620 ROI)---> 1.213 total ROI 28 points (route that I often use)16.0% armour (0.571 ROI)19.5% structure value (0.698 ROI)---> 1.269 total ROI 31 points (everything except the one skeletal that you avoided)16.0% armour (0.516 ROI)25.1% structure value (0.810 ROI)---> 1.326 total ROI(Even when) Correcting for structure only being worth ~90% of its face value (because crits), you find that the more points you invest in survival the better your return on investment. (Except for that 27-point tree, avoid that one, it's bad). All of these trees avoid one skeletal density, but I always pick that one up. I just avoided it because you avoided it, and wanted to stay consistent. =3