Level:
1-11 Normal Mode Act 1
12-18 Normal Mode Act 2
19-23 Normal Mode Act 3
24-31 Normal Mode Act 4
32-36 Normal Mode Act 5
37-43 Nightmare Mode Act 1
44-48 Nightmare Mode Act 2
49-52 Nightmare Mode Act 3
53-62 Nightmare Mode Act 4
63-73 Hell Mode Act 1
74-80 Hell Mode Act 2
81-83 Hell Mode Act 3
83-94 Hell Mode Act 4
95-99 Hell Mode Act 5

These are the ideal leveling places for the level character.