https://umod.org/guides/rust/basic-concepts-of-gui#ui-position

So from the link above I am trying to solidify my understanding of offsets and anchors

I tried creating a rectangle with an anchor of 0.5 0.5, 0.5 0.5

And an offset of -640 -360, 640 360
based on 1280x720

Which I feel should have made a full size container (yes, i know i can just use 0.0 0.0, 1.0 1.0 to "fill" the space) however I am trying to grasp what with pixel offfsets do with different screen resolutions. I would have assumed this would have taken up the space regardless of the resolution of the player.

But my resolution is 2560 x 1440 and when I did the above, this is what I get on the blue box. Does the offset not scale with resolution or am I thinking about this incorrectly?

X and Y offsets scale together so that a given element will maintain its aspect ratio regardless of resolution. This can result in a sort of "black bars" effect outside the element like when a movie's picture doesn't fit the screen's aspect ratio. The size of the bars depends on the resolution.

yea, but i figured it would scale from 1280x720 like the documentation said. 

I had to set my offsets to be 1828 x 1028 to "fill" the screen. on 2560x1440

But I had figured that given my width ratio to be .5 (2560 / 1280) and my height ratio to be .5 (1440 / 720), so if I would have specified a offsetmin of (-640 -320) and an offsetmax of (640 320) which would in turn become a size of 1280x720 X 2 = 2560 x1440. But what I am finding is this isn't the case and I am failing to figure out what it is actually doing. I understand that it is trying to keep an aspect ratio, but what I am failing to figure out is what 1280x720 from the documentation means other than that is when 100x100 is true 100x100, and if I would ever use offsets for a UI that would want to scale, or if I should just use anchors as they are pure percentages. 

Even when my resolution is 1280x720, if i use 640 360 it doesn't fill the scree

using Oxide.Game.Rust.Cui;
using UnityEngine;

namespace Oxide.Plugins
{
    [Info("UITests", "celticbagger", "0.0.1")]
    [Description("UI Tests")]
    public class UITests : RustPlugin
    {
        private const string UI_NAME = "HelloWorld";
        //private const int SCREEN_WIDTH = 1828;
        //private const int SCREEN_HEIGHT = 1028;
        private const int SCREEN_WIDTH = 1280;
        private const int SCREEN_HEIGHT = 720;


        #region json
        private static string TEMPLATE = @"
        [
          {
            ""name"": """ + UI_NAME + @""",
            ""parent"": ""Hud"",
            ""components"": [
              {
                ""type"": ""UnityEngine.UI.Image"",
                ""color"": ""1 1 1 1""
              },
              {
                ""type"": ""RectTransform"",
                ""anchormin"": ""0.5 0.5"",
                ""anchormax"": ""0.5 0.5"",
                ""offsetmin"": """ + SCREEN_WIDTH / -2 + @" " + SCREEN_HEIGHT / -2 + @""",
                ""offsetmax"": """ + SCREEN_WIDTH / 2 + @" " + SCREEN_HEIGHT / 2 + @"""
              },
              {
                ""type"": ""NeedsCursor""
              }
            ]
          },
          { 
            ""name"": ""f96a-e995-af1d"",
            ""parent"": ""HelloWorld"",
            ""components"": [
              {
                ""type"": ""UnityEngine.UI.Text"",
                ""color"": ""1 0 0 1"",
                ""fontSize"": 14,
                ""align"": ""MiddleCenter"",
                ""text"": ""{labeltext}""
              },
              {
                ""type"": ""RectTransform"",
                ""anchormin"": ""0.31 0.593"",
                ""anchormax"": ""0.695 0.707""
              }
            ]
          },
          {
            ""name"": ""8f66-03f8-9c3a"",
            ""parent"": ""HelloWorld"",
            ""components"": [
              {
                ""type"": ""UnityEngine.UI.Button"",
                ""command"": ""guitest.close"",
                ""close"": """",
                ""color"": ""0.31 0.31 0.31 1""
              },
              {
                ""type"": ""RectTransform"",
                ""anchormin"": ""0.385 0.093"",
                ""anchormax"": ""0.635 0.427""
              }
            ]
          },
          {
            ""name"": ""a7dd-4c1d-71fb"",
            ""parent"": ""8f66-03f8-9c3a"",
            ""components"": [
              {
                ""type"": ""UnityEngine.UI.Text"",
                ""color"": ""1 1 1 1"",
                ""fontSize"": 14,
                ""align"": ""MiddleCenter"",
                ""text"": ""{buttontext}""
              },
              {
                ""type"": ""RectTransform"",
                ""anchormin"": ""0 0"",
                ""anchormax"": ""1 1""
              }
            ]
          }
        ]
        ";
        #endregion


        private void OnServerInitialized()
        {
            cmd.AddChatCommand("guitest", this, CommandGuiTest);
            cmd.AddConsoleCommand($"guitest", this, (arg) =>
            {
                CommandGuiTest(arg.Player(), "guitest", arg.Args ?? new string[] { });
                return true;
            });

            cmd.AddConsoleCommand($"guitest.close", this, (arg) =>
            {
                CommandGuiTestClose(arg.Player(), "guitest.close", arg.Args ?? new string[] { });
                return true;
            });

            foreach(var player in Player.Players)
            {
                CuiHelper.DestroyUi(player, UI_NAME);
            }
        }


        void Unload()
        {
            foreach (var player in BasePlayer.allPlayerList)
            {
                if (player.IsConnected)
                    CuiHelper.DestroyUi(player, UI_NAME);
            }
        }

        private void CommandGuiTest(BasePlayer player, string command, string[] args)
        {
            var labelText = "Hello World!";
            var buttonText = "Close";
            var filledTemplate = TEMPLATE.Replace("{labeltext}", labelText).Replace("{buttontext}", buttonText);

            CuiHelper.DestroyUi(player, UI_NAME);
            CuiHelper.AddUi(player, filledTemplate);
        } 

        private void CommandGuiTestClose(BasePlayer player, string command, string[] args)
        {
            player.SendMessage("Closing menu!");
            CuiHelper.DestroyUi(player, UI_NAME);
        }
    }
}

Those coordinates should fill the screen. Have you changed the UI scale on your client? Offsets follow client UI scale. If you want to bypass UI scale, there are new parent elements you can use instead, recently added.