Tetirs is right about his calculations. Word of healing healing bonus applies only if after the initial base heal + divine favor, ur target is still below 50% hp.
the best thing to do is cast it when ur target is reaching 2/10 of hp of his bar. by that way, u wont miss.
Also consider the following.
If the char u are healing has standard hp this is the scenario
Max hp 480
50% 240
lets say u heal him when he is arround 200 hp.
A full lvl 20 healir would give him a standard avg of 65-70 heal + divine favor. Lets say a 35-50 divine favor bonus for a total of 100-120 heal. Wich will bring ur target to 300 hp wich is above the 240 requirement for word of healing.
So in conclusion, the lower max hp ur target has the harder is gonna be to heal them and get the word of healing bonus.
Imagin a standard char with 2 superiors and no vigor.
his mas hp will be 480 - 150 of runes = 330.
this means his 50% lies at 165 hp. so he should have arroun 20-30 hp left wheh u heal em to get the bonus.
Word of healing works great with fertile seasons spirits or hp buff protection spell.

Makes it easy to get bonus.
