107.3. Many objects use the letter X as a placeholder for a number that needs to be determined. Some objects have abilities that define the value of X; the rest let their controller choose the value of X. 107.3a If a spell or activated ability has a mana cost, alternative cost, additional cost, and/or activation cost with an , [-X], or X in it, and the value of X isn’t defined by the text of that spell or ability, the controller of that spell or ability chooses and announces the value of X as part of casting the spell or activating the ability. (See rule 601, “Casting Spells.”) While a spell is on the stack, any X in its mana cost or in any alternative cost or additional cost it has equals the announced value. While an activated ability is on the stack, any X in its activation cost equals the announced value.
Ja das bezieht sich dann auf das gewählte X, trotzdem hat der Spell auf dem Stack die entsprechenden CMC