EDIT: Here is the formula for warforged heal amp:
((100 + HealAmp Rating)/100) x base
where base = 0.5 for WF which becomes 0.6/0.7/0.8 with healer's friend and 0.45 for bladeforged which becomes 0.54/0.63/0.72 with healer's friend.
Testing below by Eth confirmed this formula.
Thanks to mezzorco and Eth for determining/confirming the formula.