I am looking to make a homebrew feat based on the Lucky feat but using the Portent mechanic, and am trying to find the balance point on the amount of dice.
Disclaimer: I don't think Lucky is overpowered, neither does anyone at my table, but as it is the closest feat to what I am looking for I am trying to use it as the balancing point for the new feat. This is not an attempt to homebrew a powerful feat.
Starting at 2nd level when you choose this school, glimpses of the future begin to press in on your awareness. When you finish a long rest, roll two d20s and record the numbers rolled. You can replace any attack roll, saving throw, or ability check made by you or a creature that you can see with one of these foretelling rolls. You must choose to do so before the roll, and you can replace a roll in this way only once per turn.
Each foretelling roll can be used only once. When you finish a long rest, you lose any unused foretelling rolls.
The description of the Lucky feat says (PHB, p. 167):
You have 3 luck points. Whenever you make an attack roll, an ability check, or a saving throw, you can spend one luck point to roll an additional d20. You can choose to spend one of your luck points after you roll the die, but before the outcome is determined. You choose which of the d20s is used for the attack roll, ability check, or saving throw.
You can also spend one luck point when an attack roll is made against you. Roll a d20, and then choose whether the attack uses the attacker's roll or yours. If more than one creature spends a luck point to influence the outcome of a roll, the points cancel each other out; no additional dice are rolled.
You regain your expended luck points when you finish a long rest.
The Lucky feat is widely applicable to pretty much any roll, and powerful because it can be used after the result.
The Portent feature is powerful because it can force an auto-failure or auto-success, but less applicable because if the rolls are middling they are less powerful.
How many Portent dice would be balanced against the 3 points (dice) from the Lucky feat?
I am currently leaning towards 2, but that is more based on gut feeling than anything as I have neither used, nor seen in use either of these abilities yet.
Addendum: If it matters, and it might, this is intended to be used by a Divination wizard, so if adding extra Portent dice is simply too powerful, that would be an acceptable answer, although one I would have to be convinced by.