How to pre-optimize a library for Low video quality on iPad2?

I’d like to use the Optimize feature in PMS to create optimized versions of our kids’ movies that are equivalent to the “Low” sync quality on Plex for iOS. Should I use “Optimized for Mobile” or a custom quality setting for iOS?

Does anyone know the bitrates that correspond to Highest, High, Medium, and Low in the iOS app?

I can confirm that Low = 1.5 Mbps.

At first, the version of Plex for iOS I was running didn’t have the details, but after updating from iOS 8 to iOS 9 and then updating Plex, I could see the custom settings on the iPad and the details of the bitrates.