For example, say you wanted to choose a target with mana value 6. You need X to be at least 6. You could choose to cast March of Otherworldly Light by paying . You could also exile white cards from your hand to reduce that cost by for each one. If you had a reason to, you could even choose a greater value for X.

The mana value of March of Otherworldly Light while it's on the stack is the value chosen for X plus 1, no matter how much mana you actually paid to cast it.

You may exile more cards than necessary for March of Otherworldly Light's first ability, but you can't reduce the mana it costs to less than this way.