Transparency HELP

I'm having trouble with this. I'm creating an assassin game where I need a spell with three levels that gives a certain transparency (50%, 70%, 90% respectively) to the unit who casts the spell. Sort of like shadow meld, except not fully transparent. I can't figure out the triggers to do this.

Here is the main problem; I can't find the correct Condition that corresponds to the spell level. I need it like this:
(If (level of spell being cast = 1) then set unit transparency=50%)
(If (level of spell being cast = 2) then set unit transparency=70%)
(If (level of spell being cast = 3) then set unit transparency=90%)
But the condition "level of spell being cast" doesn't exist. Can someone PLEASE try this spell out for me? Rep points will be added! And I'll give you a big hug.

Well, the condition is an "Integer comparison" and it's "Level of ability being cast" not spell. But that shouldn't have been too difficult to figure out since the event should be "Unit begins casting an ability" or something. Nothing mentioned about spells, just abilities.

You could use Unit - Level of Ability(ability being cast) for Unit(casting unit)