Replace chain at 0.5% wear, or 0.75% wear?
Its up to personal preference at the end of the day, but a general recommendation is to replace at (or before) the 0.5% point for 11 speed and 0.75% for 10 speed or lower. You can also see if theres something in the chain manual or chain wear gauge's manual. 

The multiple indicators are there because some people will want to change at 0.5%, others at 0.75% and so on. For a more expensive drivetrain, it makes sense to change earlier given the insane costs associated with all the other parts (whereas with a cheap drivetrain, you can run longer).