Following our discussion on Lucky feat(ure)s (and follow-up), we’re going to run the analysis through a test case. This might seem tough with the variance in martial proficiency, typical ability score investments, and saving throws between classes. Thankfully, there’s a useful saving throw example that is relevant to every character.
Death Saves make an excellent test case, since they have an average success chance at DC 10 and they apply universally. There’s no universal rule providing flat bonuses or rolling with (dis)advantage, which limits the variables (though certain class features may provide exceptions).
Lucky Feat. By essentially giving you advantage on your death saves, the Lucky feat means you’ll be almost 25% more likely to succeed. You’re also nearly 5% more likely to roll a natural 20, which will revive you instantly.
2 Portent Dice. You have a ~10% chance that one of your portent dice is a 20. In that case, you can automatically recover if dropped into death saves. The odds of one of two portent dice being over 10, and therefore useful for a death save, is 80%.
3 Portent Dice. You have a ~14% chance that one of your portent dice is a 20, allowing you to automatically recover. The odds of one of three portent dice being over 10 is 91%, so you’re likely to get some help.
Halfling Lucky Racial Trait. The racial feature will turn a failed save into a successful save in 2.8% of situations. Where this really shines is that it should prevent you from accruing two death saves upon rolling a critical failure. Of course, there’s always the chance that you’ll roll another 1, but at 1:400 odds, it shouldn’t happen too often.
Use First vs. Roll First
Pay attention to when the feature is triggered, as that can affect the odds.
The Lucky feat allows you to use it after viewing your roll, but before knowing the result. Thankfully, with death saving throws, we know the parameters, and can tell whether a 10+ is a success without any guesswork. While this timing has no practical effect on the odds of success, it does enable the user to preserve their uses of the Lucky feat for when the dice turn against them.
The Diviner Wizard’s Portent Dice function a little differently. The class feature requires the portent dice to be used before the roll occurs.
As a result, you get some fiddly odds on the round-by-round odds to revive. Consider this: you can only revive on your 1st or 2nd death saving throw by rolling a 20. Well, if you have a portent 20 stashed, you’re likely to use it the first round (which we assume here). Therefore, the only chance you have to revive on the second save is by rolling a natural 20.
If you don’t have a 20 stashed, you’re going to roll the die or burn a 10-19 portent die for a success.
Using Portent Dice. If you assume that you use any 10+ portent dice you have before rolling, the odds are as follows:
Rolling. If you assume that you roll any time you don’t have a natural 20 in storage, you may have the odds from the portent dice, plus the odds of rolling for a success. However, it drops after that since you’re basically just taking the rolling odds.
Risking It. There’s also the strategy to delay using portent dice until after you’ve already failed a single death save. This can be a good idea to conserve portent dice, but I wouldn’t push it to two death saves since a critical fail would kill your player outright.