Larian Studios Forums Baldur's Gate III Baldur's Gate III - Suggestions & Feedback What stats mechanics define chance to hit?

Small mistake there, Mad...

With saving throws, you don't roll anything - your target rolls to save against you.

So in your grease example, the 18 int wizard has a saving throw DC of: 8 (static value) + 4 (Int modifier) + 2 (proficiency bonus) = 14
This means that the *Rogue* needs to make a saving throw that meets or beats 14.
The Rogue is proficient in Dex saves, and has a Dex of 14 as well.
This means the rogue's saving throw will be: 1d20 (their roll) + 2 (dex modified) + 2 (proficiency bonus). This means the rogue needs to roll 10 or more to beat you, and in this case, not slip over.
In percentages, this means the Rogue has a 55% chance to succeed on a straight roll, or that you have a 45% chance of landing the effect.

Against an enemy with much lower dex and no proficiency in the save, such as an ogre, the creature would need to roll a lot higher to save against your spell, which would display as a much higher chance for you to 'stick' the spell.

The game doesn't do a very good job of explaining any of this at the moment - Someone who is not familiar with 5e, or D&D, doesn't really have a solid way of understanding most of this, especially with how lacking the event log currently is.