WoW: Effect of armor

I took Raslebol to a lonely corner of the world and started removing and putting on armor pieces, each time noting the armor class and the damage reduction resulting from it. The result is plotted in this graph: This is basically what I expected, a curve with diminishing returns. If I mentally extrapolate the curve to AC 10,000, I only arrive at about 60% damage reduction.

What still remains to be done is to calculate the effect of 1% more or less damage reduction for a single combat. I have the numbers for my last Scholomance raid, where I took nearly 300,000 points of damage at 57% damage reduction. Meaning that without damage reduction I would have taken 700,000 points of damage, and each point of damage reduction was worth 7,000 hitpoints over the total raid. But I need to know the numbers for one fight, not one raid, so I can see whether for example a helmet with 200 armor more but 5 stamina less is better or worse than the alternative.