QoL Karthus Buff

Cho can get that because Feast deals true damage.

Karthus ult deals magic damage so it would be a huge pain in the ass to accurately predict whether or not it would kill the target.

^ no there's no "predict" in programming. It's a very simple formula to determine whether or not someone will die to a karthus or any magic ult, converting their health to effective health. It could be done for Karthus but that would just make him too easy as an ULT bot, and probably make him worse for beginners.

The time to ult if your jg is ganking botlane is when they've over extended and just before they see they jungler (they aren't in lethal range yet) your team/botlane get the kill gold so they WIN their lane (rather than you getting the gold) and you get assist gold staying ahead of your lane opponent although not winning the lane outright just from that.