Peter,
first of all you need to be awared that your encore-trascoding- aud project-setting needs to fit with your source-material.
so it´s no good idea to have a H264-project@24fps and your assets are M2V 25fps. in that case encore will re-transcode your assetes to H264 and the dedicated framerate.
of course every rule has it´s exception – that are encore-menus. menus allways will be transcoded to 23.98 MPEG2, even if you have a H264-project, so the best way to go is to use straight MOV for motion menus.
if you are using squeeze make sure that the encoder-settings fit BD-specs otherwise the resulting MPEG/H264-files will be transcoded again as for the same reason as the menus will be transcoded as well.
accoring to target-bitrate: for a 90min-feature-film your target-bitrate it could have 25Mbit and peak-rate at approx 30Mbit.
but mainly your bitrate is depending on the numbers of your audio-tracks, too. PCM audio has 1.5MBit/s while dolby digital only has 640kb/s max.
BD-spec allows 40MBit/s for video and 48MBit/s for the entire TS-stream.
cheers
danny