Hi Kumada,
You would probably be fine with an even shorter statement that covers the bases:
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"Standard difficulty on all tests is DR 12. If a test would be at “half skill” it’s DR 15. At “fifth skill” it’s DR 18. If a PC is opposed by an NPC, only the PC rolls. NPCs with skill levels imposing half and fifth skill rolls on PCs use DR 15 and DR 18, respectively."
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Cheers,
Brian C.