It just depends on if you want it fluctuating or not. The way I described would encourage focus firing, but you could set the mitigation values high enough that you'd need to focus fire to take them down. And that could lead to some nasty synergies (say a tank like ship buffs the defense of everything but itself, forcing you to focus on them first while other vong ships do all sorts of nasty things to the enemy).
Might go this route. The shield decay bit is interesting as well, you can give it a negative value which would essentially replenish shield mitigation, though the problem with doing this is that it bypasses a ship's max mitigation, and increases without stopping, which is unfortunate.
If you just left the shield mitigation values static, it would sort of function like armor, with 50% mitigation equaling 20 armor, though keep in mind it doesn't change on a linear scale.
The non-linear aspect is what makes it interesting, and different. I'll let them be dynamic, increasing slowly from the initial 15% to the initial 30% maximum.