Utility Session Management - It's Inside the Database in Teradata 13.10! (UPDATED) - comment by carrie

You don't need to  calculate the number of sessions yourself.  The purpose of session management is to do that for you in the database.  The formula that session management uses to determine the default number of sessions for ARC is the following:
 
If # AMPS > 20 ==>   4 + (#AMPs / 50)
 
In your case that would be about what you are using now for the medium data size , 8 or 9.  Session management will divide that by 2 to determine the number of sessions for the small data size (that would be 4) and multiplies it by 1.5 to determine the large data size (that would be 1.5).
 
I cannot tell you the ideal number of sessions.  I suggest you use the default settings, and adjust them if you need to.   It sounds like the defaults are close to what you are already using anyway.   All of this is documented in  TASM orange book, so you might look there if you need more detail on session management. 
 
Thanks, -Carrie