Efficiency will adjust how long your sprinklers run. Let's say based off all your settings, Rachio wants to run Zone 1 for 10 minutes. If you change your efficiency down to 50%, you are telling the system that your grass is really only getting about 5 minutes worth of water, and it'll water 50% longer.... but don't hold me to those numbers exactly, I'm just guessing.
If you have 100% efficiency, then the Rachio will only water as long as the settings determine (but no one gets 100%). I got my efficiency readings from using my Orbit catch cups which automatically give you that info.
I thought about using 9" for bermuda, but my run times were pretty long, and I never reached 9" last summer, so I stuck with 8" for now.
I'm using the 0.65 now since the rye is pretty much gone.