Looking for someone good at math to assist with a build

I'm trying to figure out at what MR i will deal more damage with a void staff with kayle than with a rabadon's with this build. nashor's runaan's zephyr (what I plan on switching my boots for) zhonya's Abyssal scepter

as far as the math is concerned, -20 from abyssal -15% from kayle's passive (assuming full stacks for this test) -6% from devastating strikes, or 39% with devastating+void staff -then my hybrid reds

and i'm aware of rabadon's+void staff giving higher dps than if I had abyssal, but I use that item unless the enemy has no AP threat. Anyway, if anyone skilled at math or programming can help me figure out at what MR threshold kayle will deal more damage with void staff than rabadon's, that would be great.