If you are 75% good, your counterpart will be 75% evil.
If you are 99% good, your counterpart will be 99% evil.
What do you do if you meet your evil counterpart?
If you are 75% good, your counterpart will be 75% evil.
If you are 99% good, your counterpart will be 99% evil.
What do you do if you meet your evil counterpart?
I can fix her.
I just realized there was never an evil twin episode in Buffy.
The Wish (s03e09) and Doppelgangland (s03e16) are pretty close.