You add cost increases to it, and subtract cost reductions from it, to arrive to the final cost
How much do Disney Lorcana cards weigh
For exactly the reason that so many people have to ask "Why thirteen?"
Now, Blue-White Yorion will have Hengegate Pathway to fix its mana, as well as an excellent foretold package that includes Saw it Coming and Doomskar

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So now, the converted mana cost question is simple: if Destined to Lead isn't on the stack, it has a converted mana cost of 6