COMPARTMENT INITIALIZATION: A_0

Dear Group, I am probably revisiting this issue. I am working on a study where the blood samples are collected at steady-state. Subjects are samples for pre-dose blood sample and 4-6 samples after clinic observed doses (typical way). The prior-dose to the clinic visit was not recorded. I would like to use the A_0 option to estimate the initial central compartment amount as a parameter along with BSV. I set SS=2 at the dosing event so that the compartment amounts not to be reset to zero but that the compartment amounts are to be set to the sum of the steady-state amounts resulting from the given dose plus whatever those amounts would have been at the event time were the steady-state dose not given. Can somebody explain how NONMEM actually calculates the A_0 of a compartment when SS=2 or SS=3 given and also comment on the information does that parameter give more than the trough concentration measured at the clinical visit? Regards, Ayyappa