I would rather have like 50% min radius, so that other shield bearers and forcefields can start "picking up the slack". However, I can see good arguments for both near 100% min and near 50% min, and both would be useful in different situations. I'm willing to hear other arguments first.
I'm thinking something like 90% would be tactically ideal: that way a stack of spirecraft shieldbearers can "pass it around" themselves, but they'll all go out before the normal stuff starts taking hits. If you don't want the spirecraft ones to take fire because you think your normal shields can handle it, just put the unrepairable ones in low-power.
On the other hand, maybe 100% is the best: do you want your spirecraft shields spreading out the damage amongst themselves? Or do you want them to go down one by one so that afterward you may have lost some completely but the remaining ones have higher % health left? Seems like you would want that so you could reclaim the cap and build more (if desired) rather than having a lot of lower health ones sitting around.
If you do implement a <100% minimum (call it M%), would hit M% radius at M% health (and then not shrink any further), or at 1 HP left (so that it keeps shrinking all the way, but still never below M% radius before it dies)?
The latter, it just changes the lower end of the interval, not the fact that the slope is linear from one end of the interval to the other.
EDIT: Oh, and if we do go with 100% min radius (or M% radius at M% HP) for spirecraft shield bearers, the dynamic coloring would work well for that, though I don't know how much of a pain it would be to code, and how much GPU/CPU load it would take.
I don't really want to try to do a diffuse on these, I don't think it would work well with the pretty distinctly purple graphics. But I could be wrong. Adding diffuse is a mild increase on the GPU and thus not something I'd leap to do, but probably nothing compared to the additional cpu cost of having movement-blocking forcefields in the system.