I heard so many times, that if I wear mana shield - damage I took on myself doesn't count. Sounds logical. The same when I dealt the damage to the boss, which healed him up because of the element I used (for example I used SDs on The Pale Worm) - it also doesn't count. I noticed that people decide to take off their armors during the fight with the boss, to get more damage and increase their participation. That's why I suppose that the only damage dealt by a boss that count as a participation is the damage you get on yourself in fact, so from your example - 20 hp in case of wearing SSA. Unfortunately as Severblade said - we can not be sure.