Assume 80% chance to drop slot 1-5, 2% chance for artifact, 2% chance for insignia, 2% chance for ring, 2% chance for core, 2% for special roll (are uru special?)
So 80% base chance for slot 1-5 on first roll
Assume 1% chance for unique so .01 *.80 = .008 or .8% chance for a unique on...