You need to consult the System Builder Redbook or Redpiece for the CPW rating for your machine. That will be based upon the model, processor feature, tserver feature and number of activated processors out of the actual number of processors included in the machine for some COD (Capacity on Demand) systems.
In the redbook SG242155
IBM System i5, eServer i5,
and iSeries System Builder
IBM i5/OS Version 5 Release 4 – January 2006
For a model 550 it has 3300 to 12000 CPW for processor feature 8958 and server feature 0915. The model 550 is a 1 to 4 processor machine. The more processors that are activated, reduces slightly the actual CPW available. That is why you have 3300 CPW for one processor but only 12,000 CPW foe 4 processors.
If you have one processor on this machine that is activated and you carve that up into 3 LPARs with 50% for LPAR 1 and 25% each for LPARs 2 and 3, you would simply divide 3300 / 4 and LPAR 1 would get 2/4 or 1/2 for 1650 CPW while LPARs 2 and 3 would each get 825 CPW. This is the batch rating.
For interactive CPW, this depends on the model system and what the system builder shows you have for interactive and then you assign a % of the total interactive CPW to each LPAR.
I hope this helps answer your question.
Remeber the System Builder redbook or redpiece will show you all that you need and how to get the processor and server features.