If you spend the mana added by Helga's last ability on a creature spell with in its mana cost, the mana can be spent on any part of that spell's total cost. You're not limited to spending it only on the part.

Helga's last ability is a mana ability. It doesn't use the stack and can't be responded to.

If a spell has in its mana cost, use the value chosen for that X to determine the mana value of that spell.

You don't have to spend all the mana added by Helga's last ability on the same spell.