If a damage buff or debuff (e.g. exhaust) is applied to a champion while they are channeling a spell (after they started the channeling) like ww' ult, kata's ult or fiddle's ult is the damage for that spell adjusted mid-channeling, or the buff/debuff has to be applied before they started channeling in order to take effect for that spell damage. What about spells that involve traveling projectiles like ez, ashe, draven, ziggs ulties? Or delayed spells like leona's ult?
I ask this as I mainly play support and when I see one of those ults, if I can't cc to stop them, I apply exhaust while they are channeling to lower their ult's damage for the remaining of the channel and I was just wondering if that works or I am just wasting my summoner's spell.
Also, as a side-note, how do you even test for something like this, or where do you get an (semi-)official answer?