The idle mixture is very sensitive to changes in injection time. Idle injection times are usually around 1.5 to 2.5 ms. If the injection time at idle is much lower than this, it may become difficult to set accurate idle and cruise air/fuel ratios. When using VE Tuning method, the VE is typically around 40% to 50% VE at idle for a typical standard engine. This may vary with modified engines. Modern engines with factory cams should idle comfortably at 14.7:1, while older engines or engines with aggressive cams may need to idle richer than 14.7:1. If the engine is hunting at idle, then the mixtures are not stable across the load and RPM that the engine is hunting through. Watch the movement of the table pointer carefully and tune out potential lean spots.
If the manifold pressure is fluctuating excessively when using MAP for determining load, then it may be necessary to use the Zero Throttle Map. Keep in mind that MAP will fluctuate as a result of unstable mixtures, so establish which is cause and which is effect before jumping to the conclusion that Zero Throttle Table is necessary. Typically, engines with aggressive cams will require Zero Throttle Table to stabilise idle.
Remember that the ECU interpolates against both RPM and load. If the engine is idling at 800 RPM, then the injection time is computed as 60% of the value from the 1000 RPM range, and 40% of the value from the 500 RPM range, so both ranges would have to be adjusted to get the correct mixture. Similarly, injection times are interpolated across load cells, so it may be necessary to adjust several cells across the columns to get mixtures close to the desired mixture, then fine tune each cell thereafter.