Here's how healing enhancements stack:

1) Your best item and best enhancement stack additively. Example: Life Magic 4 (40%) and Greater Devotion 6 (40%) add to an 80% boost.

2) Your metamagic feats stack additively. Example: Empower Spell, Maximize Spell and Empower Healing give 50%, 100% and 50% respectively - total of a 200% boost. (For Heal and Mass Heal, only Empower Spell works, and I know very few FvS's have Empower and Empower Healing).

3) These two factors stack multiplicatively. So in this case, +80% means x1.8, and +200% means x3.0, so a total of 1.8x3.0 = 5.4 times the healing output.

4) This factor stacks multiplicatively with every source of Healing Amplification the target has, except the item named Finger Necklace (this stacks in strange ways, and is probably bugged). If the target is a Warforged with Healers Friend 2 (x0.7), and a 10% and a 20% boost to Healing Amp on their DT docent (x1.1 and x1.2), then this factor is 0.7x1.1x1.2=0.924, multiplied by 5.4 (from part 4) for a grand total of x4.9896.

5) Finally, this stacks multiplicatively with any critical heal that may be rolled up.