Well blessed spiritshot / soulshot increases damage by 100% (double damage), meaning 100% (base) * 2 = 200%

+20% spiritshot / soulshot damage means increasement of bonus damage, not total damage, thus:

100% (base) * 2.2 (bonus 120% damage) = 220% damage

Difference between 200% dmg and 220% dmg is

**10% increasement** in overall damage.

(just for clarification

)