First off forgive me if this has been raised before. While reading through Desertfall, I came across this passage on determination usage:
Automatic 1: Before rolling, choose one of the d20s in your dice pool: that die is considered to have rolled a 1, and does not need to be rolled. It thus scores a critical success automatically.
In Star Trek Adventures we have this approach (referenced from the Klingon Core book):
Automatic Successes: You can spend 1 Determination to add a single bonus d20 to your dice pool that has already rolled a 1, generating 2 successes automatically. This must be the first d20 you buy using either Determination or Momentum.
The primary difference here is in Dune we change the result of a pool D20 to a 1, in STA we add one die to the pool with result 1.
Is this a known difference in Determination mechanic between the two games?
Yes. In the upcoming Achtung! Cthulhu 2d20 the new rule for Determination (called Fortune there) is used, too.
In earlier 2d20 RPGs you ADDED a die set on a “1”, in the newer ones you REPLACE a die with one set to a “1”.
(Edit: it was explicitly called a “bonus d20”, counting against the limit of bonus d20s in a pool)
I wouldn’t use it for games that originally didn’t have the “replace, don’t add” rule.
Every 2d20 game has it’s own gameplay feel, and so I find the add rule perfectly fine for MC3, Infinity or Conan.
Another balance might be that in Dune at least you can use Determination after the roll to reroll any number of dice. If other variants don’t that would be a balance for the add/replace. I’d have to check that as I’m only guessing about the other versions as I’ve not double checked.
This is how the A!C quickstarter handles Fortune (the equivalent of Determination there), too.
You can spend it after the roll to re-roll d20s or damage dice - any number of them.
In our quickstarter game, this felt alright, because A!C characters are not supposed to be that ridiculously competent as characters in other games. It helps for a more “down to earth, but still with some heroic luck” feeling.
The major turn-off was, that you can only spend ONE, a single Fortune point PER SCENE. That includes combat scenes, which makes having the opportunity to re-roll damage dice or don’t die a no-brainer, so no one would use any fortune unless they are about to die.