I loaded up my game, walked up to the first Old Knight in Heide's, and let him hit me with his 3 attack combo totally naked and then again in Havel's. I was recording both times, and I used photoshop to measure the number of pixels my HP bar went down by in Havels vs naked, and then measured the total number of pixels of the HP bar (because unlike DkS 1, I don't know a way to find current HP values ingame, only max HP). When naked, my HP was at 68% of max after attacks, with full Havel's my HP was at 76% of max, a difference of 8% of my total HP or 150 HP, since 8% of the bar is 150 HP on that character.
I used un-upgraded Havels because it was all I had, and compares best against totally naked. I would like to compare fully upgraded Havels vs fully upgraded Tseldora but I don't have that yet (it is disingenuous to compare fully upgraded Havels vs non fully upgraded alternative, naked was just done as an absolute baseline).
I can upload the videos if you want. But many people assume armor does a whole lot more in this game than it actually does, due to the static nature of damage reduction. I just went and physically tested this and these are the exact values I found, if you want to test and find different values and let me know where I went wrong I would like it, I really wish armor is actually better than I am giving it credit for.
Again, armor is MUCH more useful in PvP than it is in PvE. I swear by heavy armor when I PvP, this comparison is purely against computer enemies.