Now that we can see the real numbers for this, it's worth pointing out that the information provided to us by the devs about diminishing returns points is incorrect.
The way that it works is that the diminishing returns kick in when you boost your resistance by more than 30% of its base value. The base value of a resistance is the value obtained from either the shield generator or the bulkheads *including modifications*.
https://coriolis.edcd.io/ has been updated to use the new calculation system, and shows the correct resistances in its 'Defence Summary' panel.