I'm not a druid, but I'm also on the add tank in P3. My best advice is to stay with the tank, like within just a few yards. This should keep you from being blocked by fire. Remember, your job is to keep that tank up, so if you were helping with the raid at all at this point, you should probably stop doing it and reserve all of your resources for your adds tank and yourself. Remember to use personal mitigation CDs such as Barkskin to soak unavoidable incoming damage (this will help save you a bit of mana as well).
For mana, try to recognize early when you will be short and call it out in vent. You can rotate a healer out to heal the adds tank while you go do light raid healing (or possibly no healing) while you regen some mana back. (It might be best to swap to another healer for the adds tank this late anyway, not just for mana concerns, but external CDs used on the adds tank become very important, and resto druids do not have any)
"Faith is the assured expectation of things hoped for, the evident demonstration of realities though not beheld."