Gotta multiply it by two though, since you need a round trip from your computer -> Stadia -> your computer to get feedback right?

Ping is measured round-trip. So the game should get your input in ~8ms, process it, then it will take ~8ms for the feedback to return to you.

Depending on how user input is processed in a game, this problem can be mitigated.

Oh wow, never realized this..but I guess ping in one direction is hard to measure anyway since it'd require sync'd clocks. TIL!

Not just hard, impossible: https://en.wikipedia.org/wiki/One-way_speed_of_light#Theorie...