>It seems as if the points are purely additive and then the
>resistance is computed from the point total according to some
>formula.

And there's the rub...it figures out what your percentage resistance would be (with diminishing returns), and then subtracts the vulnerability from it. So a vulnerability is always bad, and much harder to cover up.

>Based on the values you published (20 pt = 20%, 50 pt = 40%,
>70 pt = 50%) a piece of gear granting 33% resistance should
>have around 35 points.

Those numbers are accurate, 38 points = 33%.

>If that's the case, then two such pieces of gear would be
>worth 70 points, which is 50% resistance.

50% resistance, and then you subtract 33% vulnerability from it and you get 17% resistance.