There are separate controls on the encoding engines when you choose Alternate Movie-Only output. The main controls are intended for Blu-ray/AVCHD output. BD Rebuilder will always use some form of variable bit rate encoding, depending on what you choose. There isn’t an option for constant bit rate, and constant rate factor is entirely different in the way it works.
If you want a particular size, it is always best to choose a two pass encode. This insures the greatest accuracy in output size. Jdobbs has given us an option for CRF encoding and a target size, which is what you’ve used, but it is a compromise in the way the X264 encoder functions, with a quick and relatively inaccurate first analysis before going forward with the real work of encoding.
If the output size is the primary concern, always use a two pass encode.