Given a position, how do I figure out Pixel's Per Unit?

Question

I have a custom editor for my scene which demonstrates the playable radius (how close my game ships will get to an object before slowing. The problem is that the circle draws a single line, and I want it a little thicker, say 3 pixels wide.

I can set that with this for now:

         Handles.DrawWireDisc(transform.position, Vector3.up, Radius);
         Handles.DrawWireDisc(transform.position, Vector3.up, Radius-.05f);
         Handles.DrawWireDisc(transform.position, Vector3.up, Radius-.1f);

But if I zoom in or out, then the lines become just 1 line, or spread apart, loosing the effect. I'm looking for away to guarantee the next radius will be 1 pixel out.

Answer 1

Not pixel perfect but does the job (keeps constant size):

 using System.Collections.Generic;
 using UnityEngine;
 
 public class GlGizmo : MonoBehaviour
 {
     [SerializeField][Range(3,32)] int _lineCount = 12;
     [SerializeField][Range(0.001f,100f)] float _radius = 1.5f;
     [SerializeField][Range(0.001f,1f)] float _unitWidth = 0.1f;//width when distance to camera is 1.0
 
     void OnDrawGizmos ()
     {
         System.Func<int,int,Vector3> CalcPoint = ( i , max ) =>
         {
             float f = i / (float)max;
             float angle = f * Mathf.PI * 2;
             return new Vector3( Mathf.Cos(angle)*_radius , Mathf.Sin(angle)*_radius );
         };
 
         float dist = Vector3.Distance( Camera.current.transform.position , transform.position );
 
         GL.PushMatrix();
         GL.MultMatrix( transform.localToWorldMatrix );
         ( new Material( Shader.Find("Sprites/Default") ) ).SetPass(0);//replace material to customize render style
         GL.Begin( GL.QUADS );
         {
             //set color:
             GL.Color( Color.white );
 
             //warm up:
             {
                 Vector3 A0 = CalcPoint( 0 , _lineCount );
                 Vector3 B0 = A0 + A0.normalized*_unitWidth*dist;
                 GL.Vertex( A0 );
                 GL.Vertex( B0 );
             }
 
             //segments:
             for( int i=1 ; i<=_lineCount ; i++ )
             {
                 Vector3 A = CalcPoint( i , _lineCount );
                 Vector3 B = A + A.normalized*_unitWidth*dist;
                 
                 if( i%2==0 )//even
                 {
                     //end started quad:
                     GL.Vertex( A );
                     GL.Vertex( B );
 
                     //start new quad
                     GL.Vertex( A );
                     GL.Vertex( B );
                 }
                 else//odd
                 {
                     //end started quad:
                     GL.Vertex( B );
                     GL.Vertex( A );
 
                     //start new quad
                     GL.Vertex( B );
                     GL.Vertex( A );
                 }
             }
         }
         GL.End();
         GL.PopMatrix();
     }
 }

Given a position, how do I figure out Pixel's Per Unit?

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