Many systems display the service units used as part of the normal accounting process -- so why bother calculating it?
MSU depends upon the processor rating (SU_SEC); without knowing this number you cannot do any calculations. You can find this value in the type 70 SMF records. CPU seconds times SU_SEC gives you the rough MSU.
Be aware that MSU ratings can vary 30% on the same machine, depending upon the characteristics of the workload being measured. So measuring two different jobs that do the same thing in different ways could result in different MSU values.
Also be aware that using zIIP and zAAP processors (for Java work or DB2 work) affects the calculation; I don't recall the exact details (not enough coffee intake yet this morning).
If you have MXG, MICS, or another such tool you can get such data directly from them -- which is a better source than doing calculations.