how many channels are using???
whatz ur filesperset ????

in general we need to allocated 4 buffers per file.
each buffer is db_block_size * multi_block_read_count

for eg:

db_block_size=8 and multi_block_read_count=8 then 64k per buffer

64k * 4 * 2(channels)

This config works fine for me.Rman uses large_pool_size properly

regards,
Vinod