Adds tools to the Font inspector to let you lay out the character rects in a grid.

FontEditor.cs

using UnityEngine;
using UnityEditor;
using System.Collections;

[CustomEditor(typeof(Font))]
public class FontEditor : Editor
{
    Font m_Font;
    
    static bool m_TilingControlsVisible;
    static int m_FirstCharacterIndex = 32;
    static int m_TilesU = 1;
    static int m_TilesV = 1;
    
    void OnEnable()
    {
        m_TilingControlsVisible = EditorPrefs.GetBool("FontEditor.TilingControlsVisible", false);
        m_FirstCharacterIndex = EditorPrefs.GetInt("FontEditor.FirstCharacterIndex", 32);
        m_TilesU = EditorPrefs.GetInt("FontEditor.TilesU", 1);
        m_TilesV = EditorPrefs.GetInt("FontEditor.TilesV", 1);
    }
    
    void OnDisable()
    {
        EditorPrefs.SetBool("FontEditor.TilingControlsVisible", m_TilingControlsVisible);
        EditorPrefs.SetInt("FontEditor.FirstCharacterIndex", m_FirstCharacterIndex);
        EditorPrefs.SetInt("FontEditor.TilesU", m_TilesU);
        EditorPrefs.SetInt("FontEditor.TilesV", m_TilesV);
    }
    
    public override void OnInspectorGUI()
    {
        m_Font = (Font)target;
        
        EditorGUILayout.BeginVertical("Box");
        m_TilingControlsVisible = EditorGUILayout.Foldout(m_TilingControlsVisible, "Tiling Controls");
        if(m_TilingControlsVisible)
        {
            EditorGUILayout.BeginHorizontal();
                m_FirstCharacterIndex = Mathf.Max(0, EditorGUILayout.IntField("First Character Index", m_FirstCharacterIndex));
                
                string character = GUILayout.TextField(((char)m_FirstCharacterIndex).ToString(), GUILayout.Width(20));
                if(character.Length > 0)
                    m_FirstCharacterIndex = character[0];
            EditorGUILayout.EndHorizontal();
            
            EditorGUILayout.BeginHorizontal();
                EditorGUILayout.PrefixLabel("Tiling");
                m_TilesU = Mathf.Max(0, EditorGUILayout.IntField(m_TilesU));
                m_TilesV = Mathf.Max(0, EditorGUILayout.IntField(m_TilesV));
            EditorGUILayout.EndHorizontal();
            
            if(GUILayout.Button("Configure Character Rects"))
            {
                if(EditorUtility.DisplayDialog("Configure Character Rects",
                    "This will erase all character rects and set them up again from scratch. Are you sure?",
                    "OK", "Cancel"))
                {
                    ConfigureCharacterRects();
                    EditorUtility.SetDirty(m_Font);
                }
            }
        }
        EditorGUILayout.EndVertical();
        
        EditorGUILayout.Separator();
        base.OnInspectorGUI();
    }
    
    void ConfigureCharacterRects()
    {
        CharacterInfo[] characters = new CharacterInfo[m_TilesU * m_TilesV];
        
        float sizeU = 1.0f / m_TilesU;
        float sizeV = 1.0f / m_TilesV;
        
        for(int v = 0; v < m_TilesV; ++v)
        {
            for(int u = 0; u < m_TilesU; ++u)
            {
                var character = new CharacterInfo();
                character.index = m_FirstCharacterIndex + u + v * m_TilesU;
                character.uv = new Rect(u * sizeU,  1.0f - sizeV - v * sizeV, sizeU, sizeV);
                character.vert = new Rect(0, 0, 1, -1);
                character.size = 0;
                character.width = 1.0f;
                characters[u + v * m_TilesU] = character;
            }
        }
        
        m_Font.characterInfo = characters;
    }
}