I've been working at figuring out how arpen/debuffs stack today and have a theory for a formula:
where:
A = the half armor value at 80, 15232.5
B = base boss armor, 10643(the value I found from testing)
x = sum of arpen buffs(mace spec/battle stance/gear arpen)
y = product of arpen debuffs(sunder/ff) (1-.95*.8)
Currently arpen buffs are not working as specified, and are all .81 of what they should be. I.e. Battle stance = 8.1% reduction, mace spec = 12.15%, and 20% arpen from gear(per the tooltip) = 16.2%. The nice thing is that they do stack together additively. So, if you had Mace spec/Bstance/20% arpen, x would be .81*(.15+.1+.2) = .3645
Sunder/FF on the other hand stack together multiplicatively, so 24% reduction when both are applied.
The debuffs and buffs together however do not interact as I'd expect at all. The buffs are summed together, but then do not stack multiplicatively or additively with the debuffs, but somewhere in between. After fiddling with the numbers for a long time and trying to discover a relationship, the ratio in between seems to be A/(A+B), the armor reduction for a boss at full armor oddly enough.
To hopefully be a little more clear.. if you had 10% arpen from gear(ignore the .81 bug for now) and Sunder/FF up.
expected armor reduction from arpen alone if fully multiplicative = .1 * (10643*(1-.24)) = 808.868
expected armor reduction from arpen alone if fully additive = .1*10643 = 1064.3
Actual armor reduction from arpen = 808.868 + (A/(A+B))(1064.3-808.868) ~= 959.24, which puts the final armor value at 10643*.76-959.24 ~= 7129.44, which becomes (15232.5/(7129.44+15232.5)) = ~= 68.11% damage, which is the answer you'll get if you plug all those numbers into the equation listed above.
If anyone else's results don't check out with this equation, feel free to jump in.