I had tested Shadow Mantle a while ago and had it at 1/16(6.25%) proc rate. Lately I've been hearing a lot of people say it's much higher. I finalli decided to lot one for my char, so I ran a test to re-confirm:
Test 1: 111/1638 = 6.78%(Original Test on Puks)
Test 2: 84/1521 = 5.52%(Crabs)
Test 3: 126/2080 = 6.06%(Crabs)
Total: 321/5,239 = 6.13%
For the test I used PLD/WAR, rounded up the 5 Puks in Den of Rancor and super tanked them while I spammed cure on my BRD + WHM. (Subsequent tests were done on crabs in Kuftal.)
(My older Shadow Ring tests have the proc rate at 1/8.)
EDIT: Updated additional testing~