Is this something you need to do in PBM games?
Normally, I wait until after growth has been added before switching lux on.

We get one turn "grace" period before we will need to do this.

Yup, but it was did before growth judging by the Played turns thread.

I wondered about raising that as well. Provolution has said we don't have to hurry to hook up the ivory since we're not going to be growing due to the worker about to appear. So I was surprised when we raised the lux slider. That's the turn we don't have a screen shot for the city screen.

Ahead of city expansion on emperor level, size 1 to 2, we get riots with no garrison, unless we amend luzes by a measle 10 %. It worked fine.

Well, not unless this is different in the MP games than in the 1-player.
You can always wait with increasing lux until the city has already grown, it will not riot until the next IBT.