Custom LODGroup Inspector with fields for entering distances instead of screen size percentages.

LODGroupCustomEditor.cs

using UnityEngine;

namespace UnityEditor
{
	/// NOTE: This needs to be in an editor folder with an Assembly Definition Reference referencing
	/// the UnityEditor.UI assembly. This is because it uses internal extension methods which are inaccessible
	/// outside of UnityEditor.UI.
	[CustomEditor(typeof(LODGroup))]
	internal class LODGroupCustomEditor : LODGroupEditor
	{
		public override void OnInspectorGUI()
		{
			DrawDistanceProperties();

			base.OnInspectorGUI();
		}

		void DrawDistanceProperties()
		{
			var lodGroup = (LODGroup) target;
			var lods = lodGroup.GetLODs();

			if (targets.Length == 0 || lods.Length == 0) {
				return;
			}

			if (SceneView.lastActiveSceneView == null || SceneView.lastActiveSceneView.camera == null) {
				return;
			}

			var camera = SceneView.lastActiveSceneView.camera;

			float[] lodDistances = new float[lods.Length];

			EditorGUILayout.BeginVertical();
			EditorGUILayout.LabelField("Custom Distance Tool: ");

			for (int i = 0; i < lods.Length; ++i) {
				EditorGUILayout.BeginHorizontal();

				float percent = lods[i].screenRelativeTransitionHeight;
				float distance = LODGroupExtensions.RelativeHeightToDistance(camera, percent, lodGroup.size);

				string to = i == lods.Length - 1 ? "cull" : $"LOD{i}";
				string label = $"LOD{i} -> {to}";

				float newDistance = EditorGUILayout.FloatField(label, distance);

				if (distance != newDistance) {
					Undo.RecordObject(target, "Set LOD Distance");

					lods[i].screenRelativeTransitionHeight = LODGroupExtensions.DistanceToRelativeHeight(camera, newDistance, lodGroup.size);
					lodGroup.SetLODs(lods);
				}

				EditorGUILayout.LabelField(percent.ToString("P4"));

				EditorGUILayout.EndHorizontal();
			}

			EditorGUILayout.EndVertical();
			EditorGUILayout.Space();
		}
	}
}

Reposting as a comment in case you didn't see the comment at the top of the class:

NOTE: This needs to be in an editor folder with an Assembly Definition Reference referencing the UnityEditor.UI assembly. This is because it uses internal extension methods which are inaccessible outside of UnityEditor.UI.