Backup Exec - Which IO size and R/W ratio is used or seen during backup?

Hi, 

i am sizing a new backup infrastructure.

In particular the landing zone (SAS 15K Drives).

Now i want to know which values can i

calculate with during a File Server Backup. What is happening (or how is data written at the

target)

- which IO size will be used (will be seen) during a default full backup with default settings

and NTFS on 2008 R2?

- which read write ratio during a symantec backup exec 2012 full backup 

will be measured?

With both raw values i would be able to size a good landing space.

Thanks in advance

Lucas