Yea, that’s a simple and effective way of testing it 
I remember way back we used to do that in PvP on each-other to try and figure out the formulas since we didn’t have a damage meter but then the other person could report back the health they lost.
I noticed that it changed simply cuz I looked at my status page ingame and saw that I was getting like 15-20% less reduction from wearing the same stuff as immediately prior to the update. I did not check or research it though so I didn’t know it was 500 until you posted it above, and sure enough as you can see on the picture above it is indeed 500 as I set a watch on the armor target constant So nope, didn’t know that until now either!