For the tests, I had to guess the attack rating of the mobs and then set my defense close to that number. Having done that, I let the mob hit me while I keep track of the numbers that shows up the most with and without shield blocks. These two numbers are basically the same except that one has been reduced by the shield blocks. From these two numbers, I figured out the shield absorption % with algebra.
The logic behind the testing method is that there is a flat area in the middle of the pDIF function which has 2 interesting properties. The pDIF produced is 1.0 and, more importantly to this test, this number shows up around 30% of the time under some conditions. In the test, I set my defense around the same as the mobs attack just to have this flat pDIF area present in the mobs random hit damage interval. The tests were done without engaging the mobs because the Signet defense bonus would have messed up the results.
As an example, this is the data of my last test sample.
My VIT: 85
My AGI: 59
My defense: 269
MOB: Sand Lizard in Kuftal
Shield: Darksteel Buckler (Bucker type, LV 59, DEF 14)
Shield Skill: 144
Hits taken: 42
Most frequent non-blocked hits: 11 hits for 63 damage/hit
Most frequent blocked hits: 13 hits for 44 damage/hit
Damage reduction = Between 100% - 44 / 63 and 100% - 44.99 / 63 = Between 30.16% and 28.59%
According to the equation, the damage reduction % of a Darksteel Buckler is 22% + 14 * 0.5% = 29% which is inside the [28.59 - 30.16%] range given by the data.
I didn't comment on the testing method before because it sounds complex and it's confusing if the reader doesn't understand the game mechanics concepts used.
Very interesting! What sorta of mobs were you tanking to get your shield skill ups? Also, were these done with the same shield skill or were these numbers taken while skilling?
The test mobs were Evil Weapons in the Jeuno areas, crabs and mandies in Boyahda and crabs and lizards in Kuftal. When I started, my skill was 102 and after the tests, it was 145. Although it all started as me trying to skill up shield, when I performed the tests, I kept my main focus on the accuracy of the results and the skill ups was just a side effect. I also performed some tests on the effect of AGI on the shield block rate but the results were inconclusive.