Parallel_max_servers

Currently the parallel_max_servers is set at 28 for the databse which is on a 8 CPU box. The parallel servers seem quite busy and execution times are going up for some jobs. I am worndering about increasing parallel_max_servers. Is there a formula to kepp in mind for how high I can go for an 8 CPU machine? Thanks.

A starting point would be 2*

Some things to watch for when setting this and tuning:
What are the CPU times is there a wait for cpu contention on the system?
Are you staturating the I/O subsystem?