Quoted from honorableranger7#3220 in the halo 2 discord:
Download autohotkey first, install.
Go here: https://autohotkey.com/boards/viewtopic.php?t=10159 copy and paste everything from "Library", paste it into a text editor of choice (notepad++ is recommended), then save file as "MouseDelta.ahk".
Make a new folder and put this file you just created into it.
Now go back to the autohotkey link above and copy and paste everything from "sample script 3" into your text editor and edit the "ScaleFactor" number to something between 2 and 1.5 and save it as whatever (put .ahk at end of file..)
You can always find what is ideal for you but this seems like the sweet spot for this, although, I haven't thoroughly tested it. Put this file into the same folder with the "MouseDelta.ahk" file.
Now right click the file you just made and click "Run as administrator". Run the file and test for yourself in-game.
Setting the in-game sens to 1 would be ideal to avoid pixel skipping and have smoother aiming but to each their own..
Just keep changing the ScaleFactor value, saving the script, and reloading it (from the bottom right of taskbar icons) until you find what you like best.
