Can someone tell me if my math is horribly off?

Just the core tho

Generally you want to finish an item before starting something else.

Do you need the early armor pen that BC offers but you need to start your wave clear item? If yes, get it after tiamat. If you don't need the wave clear item early, start BC first. If you don't need the armor pen early, finish your hydra first.

I would go Phage before Kindlegem because the movespeed is valuable on Renekton for both escape and engage. But otherwise it appears solid.