hmm, i should have started reading this sooner, it's very interesting.
I recall there beind shield arcs, so this should probably apply to each arc individually. And I agree on there being some kind of limited cumulative effect,
Say a weapon with damage 100 shoots at a shielded ship, lets say for calculation ease that all hit occur within generally the same timeframe.
First hit, and the shield arc gains 100 damage, and 20% mitigation
Second hit, within a small time, shield arc gains 80 damage and an aditional 10% mitigation for a total of 30%.
Third hit, shield arc gets 70 damage, and an additional 5% mitigation.
etc etc.
Now the attacking ship needs to reload, so the shields recarge at a set rate, but the mitigation also drop at an exponential rate, slowly first, but faster until the mitigation is 0.
The enemy has finished reloading, so it fires again, dealing 100 damage, adding 20%mitigation. The next shot is targeted at another ship, so the mitigation drops a bit, down to 17%. the next shot hits, dealing 83 damage, and adding more mitigation (somewhere between 10 and 12% i'm not good at exponential calculus at 2330) so the next hit would be mitigated 27-29%.
All numbers are open to tweaking of course. I don't design games