Class Ruling: Wizard Feature - Portent

Home ruling:

Potent is intended as you see the future “glimpses of the future begin to press in on your awareness.” This means you know the result of the d20 so no luck point or any other feature can override it. This is the end result of the die roll period.

So, it may get complicated if the person has Bardic inspiration or Guidance or a +17 to the skill check. Cause Potent only replaces the end resulting D20 roll. The person rolling can still add any modifiers, but cannot re-roll the d20.

You only have 2 dice per day, this isn’t something that you can really abuse over and over as far as I can tell.
So, you may replace any D20 AFTER you know what the roll is but before the result. (Or If you aren’t given an opportunity to know the roll and someone just says the result you can still you your Portent.)
_{(I reserve the right to change this homebrew ruling at any point in the future.)}

From sage advice:

A wizard's Portent is meant to replace the roll—after advantage/disadvantage is applied. #DnD https://www.sageadvice.eu/diviner-wizard-portent/

From Jeremy Crawford Tweet:

The wizard: "I use Portent!" The d20 (or d20s with adv./dis.) is rolled. Portent replaces the resulting die.
HOWEVER!
Crawford also states:
“The portent die is intended to replace a d20 roll only, not any modifiers applied to it.”