The ECU runs in open loop (i.e. no fuel trim adjustments via the oxygen sensor) most of the time. This is because the stock oxygen sensor is a narrow band unit. It's only effective through a very limited range of operation--most of the time the signal it sends is worthless which is why most of the time the ECU doesn't pay any attention to it.

Assuming you're tuning with a V-AFC the general starting point for adjustment with injectors in the 450 cc/min range is -40%, but you really should hook it up to a dyno with a wide band oxygen sensor to get more precise readings.