It Really Sucks When Riot Gets it Right and then Takes it Back
For those who don't follow the PBE closely, at the beginning of the Cho'Gath changes, he had a 12% bonus health ratio on his ultimate's true damage, and the AP ratio was removed entirely.
This was the correct design, as it encourages Cho'Gath to build tank, it rewards him for building tank, it gives him a reason to be excited about his infinite scaling, and so on. But then Riot changed it and shipped it to live with a 50% AP ratio (down from 70%) and a 10% bonus health ratio.
And now, with a 100% AP ratio on his Q, a 70% ratio on his W, a small ratio on his E and a 50% on his ultimate, we get full AP Cho'Gath. And full AP Cho'Gath gets tanky and stays tanky because his stacks don't fall off and he gets rewarded for bursting people down. We have a full AP champion that only gets harder to kill the more burst he can deal.
Cho'Gath has AP ratios because his kit is old. Any updates should be looking to tune them down, not preserve them. Tanky burst champions are never healthy, even immobile ones. AP Cho players can still enjoy the free sustain on his passive (RIP mana potions) and the huge burst of his Q. We don't need his ultimate to be a point and click AP nuke that makes him invincible without so much as a Cloth armor.
Its a pretty fun build tbh. Ludens is optional and depends on how much ms your targets have.