Visual area occupied by each object in field of view

Bizarre use case: I would like to be able to get, for each object within an agent's visual field, the % of the visual field that the object occupies.

Based on the answers to this question, I managed to get all the objects within an agent's visual field. I have provided the code below. Now I need to find out how much of the visual field, each of those objects occupies.
I'm rather new to unity and C# and have absolutely no idea where to begin. Any help would be much appreciated! I have also provided an image below to aid explanation.

 using System.Collections;
 using System.Collections.Generic;
 using UnityEngine;
 using MLAgents;
 using MLAgents.Sensors;
 using UnityEngineExtensions;
 
 public class FovObjects : MonoBehaviour
 {
     public Camera cam;
 
 // Start is called before the first frame update
     void Start()
     {
 
     }
 
     // Update is called once per frame
     void Update()
     {
         string allVisibleObjects = "";
         foreach (GameObject go in GameObject.FindGameObjectsWithTag("arena"))
         {
             Transform[] allChildren = go.GetComponentsInChildren<Transform>();
             foreach (Transform child in allChildren) {
                 Vector3 screenPoint = cam.WorldToViewportPoint(child.position);
                 bool onScreen = screenPoint.z > 0 && screenPoint.x > 0 && screenPoint.x < 1 && screenPoint.y > 0 && screenPoint.y < 1;
 
                 if(onScreen) {
                     allObjects += child.name + "-";
                 }
             }
             Debug.Log(allVisibleObjects);
         }
     }
 }

This is just for demonstration purposes. The % aren't accurate.