I assume stated shield power levels remaining in a given situation work the same as present day chemical batteries, which maintain output at or near stated voltage even as they approach total discharge. So shields at say 18% can still repel the same magnitude of firepower they can at 100%, but are 82% closer to collapsing. Is this the consensus out there?
Sometimes. It seems that there are other times they give a report on the shields with a bit more data, e.g., "aft shields at 42%," and single out an area that is no longer capable of holding, then declining at a known rate when exposed to energies beyond a certain threshold. Since the shields are not always stopping 100% of the incoming in the first place, this probably refers to areas where the grid itself, or some other hardware component, has been affected.
It generally seems that the shields are supposed to be redistributing whatever's incoming around the whole system, reducing overall protection by a smaller percentage instead of letting individual areas become overwhelmed. There are times they have changed this, though, aren't there? Reinforced a specific arc at the expense of others?
Is the ship's nose always better-protected in the primary flight direction due to the navigational deflector?
Do you think "boost power to shields" refers to exceeding the spec on the generators and so forth and "overcharging" them within a known range in which they are technically exceeding the spec but very unlikely to fail?