Rundown layouts is relatively simple and functions very similarly to tweakers RundownLayout It has 8 rundown entries because that's the max amount of rundowns you can have

"R1": {
    "Enabled": false, //This enables/disables overriding this rundownDatablockID
    "RundownDatablockID": 35, //This is the PersistentID of the rundown you want to change in rundownDB
    "Tier1": [], //Tier A
    "Tier2": [], //Tier B
    "Tier3": [], //Tier C
    "Tier4": [], //Tier D
    "Tier5": [], //Tier E
    "EnableExtensionPage": true, //Enables the R6 extension page (icon aligns may or may not work)
    "ExtensionPagePos": { //Position of the feature
      "x": 0,
      "y": -459,
      "z": 0
    },
    "ExtensionPageText": "<color=orange>://EXT" //Text header for the extension page
  }

How you actually change where you expeditionIcons are placed is in this bit that goes inside of Tier1 Tier2 and so on

{
  "label": "<color=red>br<color=blue>uh", //This changes not the name but the icon text (in vanilla this is R1A1 R1B1 and so on)
  "buttonPos": { //This overrides the position of the expeditionIcon (i suggest using unity explorer if you know how the vector3 is the LocalPosition)
    "x": -450.0,
    "y": 0,
    "z": 0
  },
  "changeScale": true, //If you don't want to change the scale (for if your rundown visuals have different scales
  "buttonScale": { //This is just the scale field in unity explorer
    "x": 10,
    "y": 10,
    "z": 10
  },
  "buttonColor": { //The color of the buttons visuals (independent from text) (alpha changes the opacity)
    "r": 1,
    "g": 0,
    "b": 1,
    "a": 1
  },
  "enableHeat": false, //Enables/Disables the useless Artifact Heat text in most modded rundowns
  "heatText": "ARTIFACT HEAT: <color=orange>100%", //override for the text (you can make it say "sex")
  "statusPos": { //Position of the status button (this is the NOT COMPLETED text or it shows how many times you've cleared that level) (its the white box under Artifact Heat)
    "x": -116.378,
    "y": -37.81,
    "z": -1.36
  },
  "overrideDecryptText": true, //The decrypt text (It's the bit of text above the expedition box
  "decryptText": "<color=green>YIPPEEEE" //Text override for the decrypt text
}