Replace chain at 0.5% wear, or 0.75% wear? Why do chain wear indicators have an indication for 0.5% wear and one for 0.75% wear? Are both measurements included because there is some personal preference from mechanic to mechanic, or rather are there separate uses for each?

I find myself asking this because right now I have a chain that measures at least .5% but less than .75% and I want to replace it before it negatively impacts the cassette teeth. For what it's worth, it's an 11 speed chain.
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).