Limiting RMAN backup size

[Sona]

Hi I am doing RMAN incremental level 0 backup .The database size is about 150GB out of which used would be about 100GB.I don't have a single volume with 100GB of space.So I need to distribute the backup across several disks.   Also I need to limit the backup size in each of the volumes depending on how much space is available in those volumes. For e.g if i want to limit /disk1 to 40GB /disk2 to 10GB /disk3 to 10GB /disk4 to 10Gb How would i do this in RMAN?   TIA