The Soulknife Rogue has the following ability:
Psi-Bolstered Knack. When your nonpsionic training fails you, your psionic power can help: if you fail an ability check using a skill or tool with which you have proficiency, you can roll one Psionic Energy die and add the number rolled to the check, potentially turning failure into success. You expend the die only if the roll succeeds.
However, some DMs I know (including myself) don't have a clear DC for skill checks. For example, on a knowledge checks, the amount of knowledge given depends on the exact result of the roll. A 6 gives no results, a 13 gives a bit of information (for example, that the writing system is from this particular ancient civilisation) and a 17 gives some more (for example, that it's a tablet with funerary records).
Or, for a task like lockpicking a difficult lock, a 15 would mean that the lock is picked but that it took a long time/was quite noisy and a 20 would mean that the lock is picked efficiently and silently.
Or, for some checks, rolling too low (like under 10) gives negatively consequences, rolling above 10 but still failing means there are no negative consequences.
Then there are checks with results directly proportional to the roll, for example, negotiating for a decrease in prices gets a discount in gold equal to the number rolled if it's above 15 (so save 17 gp by rolling a 17, save 22 gp by rolling a 22).
My question is: how should the Soulknife's ability be handled ? When should they be allowed to spend psionic dice, and when is the dice expended ?
Should the rogue be allowed to expend psionic dice whenever they haven't reached the best possible results (for example, getting information from a knowledge check, but wanting some more), or only when it's clearly a failure (no results at all or negative consequences) ?
If the rogue uses a die and turns a result with negative consequences into a failure but with less drastic negative consequences, is the die spent ? Since the roll is still a failure, but it's still a less punishing failure, so the die did have effects.
Personally, I think that the die is expended if it changed the result of the skill check, but I am not sure whether it should be allowed to be used anytime.