Yes, the -b option is really designed to reference one build script. With the -c option, you can specify the version of the settings.gradle file to configure your entire multi-project build with the appropriate build files.
You’re most of the way there with what you’re already doing, but you should set the appropriate file name for your root project build script (like you are for the subprojects) in each version of the settings.gradle file instead of specifying it with the -b option.
Although these are quite old now (2012), here’s a couple threads that support this intention of the -b option and some related information: