Remember all rituals have to be cast first. Then the system roll within the description of the ritual to determine how well the ritual worked.
The roll is Int + Occult.
Average difficultes are
3+LvL
Level 1 Ritual ave diff is 4
Level 2 Ritual ave diff is 5
Level 3 Ritual ave diff is 6
Level 4 Ritual ave diff is 7
Level 5 Ritual ave diff is 8
Now mind you there will be modifiers assigned by the ST so make sure you consult an ST first. But under normal circumstances the above difficulties should be the norm.