First, to check whether Founder's Breastplate rounds or not:

Eschan Sorcerer, 3% killer rate from vorseals

Base: 303

Founder's Breastplate: 306 (+1%)

So, it looks like Founder's Breastplate rounds the damage down to the nearest percent.

Now for the meat of my tests. I did tests on both non-NMs and NMs. Arcana Killer II was the only base level killer effect active.

Sinister Seidel (non-NM)

Base: 364

Founder's: 382 (5% above base)

Circle 418 (15% above base)

Founder's/Circle: 468 (11.96% above Arcane Circle)

Circle (Enhanced): 425 (17% above base)

Founder's/Circle (Enhanced): 480 (12.94% above enhanced Arcane Circle)

Arcana Killer II on non-NMs is 10% killer rate.

14-15% rate for Arcane Circle

16-17% rate for enhanced Arcance Circle

Teekesselchen (NM)

364

374 (3% above base)

400 (10% above base)

428 (7.00% above Arcane Circle)

404 (11% above base)

436 (7.92% above enhanced Arcane Circle)

Arcana Killer II on NMs is 6%.

8-9% rate for Arcane Circle

10-11% rate for enhanced Arcane Circle

Due to the breastplate's flooring effect, I can't hammer down the circle effect further with these tests. Also, I don't have the numbers written down anymore, but they're much simpler tests due to not involving Arcane Circle, so I'll put this conclusion down too:

Killer effects from food are halved on NMs.

Tests to be done: check whether vorseal killer rates are nerfed on NMs (I've confirmed that they're not halved, but 3% > 2% is still possible with my numbers), and to use an odd value source of killer rates to nail down circle effects more exactly. ]]>