I would like to know if there is a way to modify the maximum limits of the values.

For exemple create a custom reward with a item which would have (independently) ~0.2% of get, and not knowing exactly the mechanics I assume that the chances are basically limited to 0.5% of get for a value of 100/-100.

So is it possible to make a maximum value of 200/-200, or to double these values (for example a value of 75 would become the equivalent of a value of 150), or even to implement a value % instead the "brute" value?

Oh ok, I found thx you

and how to change the grab of a reward?

There is not a way to change the min and max values. The chance value of a cube is chosen on a normal graph from centered at 0 from -100 to 100. Thus the odds aren't as straight forward as 0.2%. You can however change the range the cubes grab rewards from. I forget what the default is, but if the cube has a value of 15, it will grab any rewards within the specified range, so if the min and max range values are 20, then a cube with a 15 chance value can produce a reward with a chance value of -5 to 35. Hope this made sense and helps

rangeMin and rangeMax in the chance cubes config

like this?

{
"chancecubes:item": {
"chance": 100,
"rangeMin": 0,
"rangeMax": 10,
"isGiantCubeReward":true,

"Item": [
  {
    "item": {
      "id": "mod:item_name"
    }
  }
]

}
}

No it's in the chance cubes mod config. It's not a reward setting