Spreadsheet showing deflection and wounding of copper bolts off dwarves wearing full uniform thickness plate armor (see thread). This spreadsheet shows the results for metal armors available in-game, and with contact area set to 2 or 10.

It appears that Urist is correct, and the momentum required for 50% serious wounds is multipled by Contact Area*LAYER_SIZE rounded to hundreds (rounding up to 100, and rounding down for numbers above 100). In this scheme, Contact Area 2 rounds to 100, and contact area 10 rounds to 200. Correspondingly, the momenta for Contact Area 10 are exactly twice those for Contact Area 2. I will eventually have to confirm this by systematically varying Contact Area.

I also ran for iron bolts with contact area 10, and got identical results when plotted vs. bolt momentum.