Here is the ignition timing map. The values you see in here are assuming that your base timing is set to 15 degrees; this is when the engine is at idle, the hard idle switch is on and the ECU is making it fire 15 degrees. You calibrate what the ECU is trying to run the engine at by turning the CAS. Once you have done this, the value the ZEM, conzult, or techtom tell you are the ACTUAL timing in degrees BTDC that it is firing the plug off based on the presumption that your base timing is at 15 degrees (by setting the CAS).