If you have a heal with 100ee and you add 20hm you will have 120ee (if not overcapped). This would be a boost of the factor 2.0 (from 100ee) to 2.20 (from 120ee). Healing improvement of 10%.
For 200ee to 220ee it would be 3.0 to 3.2 so 6.7% more.
So bigger relative effect on lower ee.
There was another bonus on top. If you have hm on you, all heals to you will have an increased effect. Didnt really test yet how much that is doing