Timing

You have probably looked at a log where the DME pulled 3° of timing and though that's not too bad then seen another log with 8° of timing pull and thought that's too much. Equally, you might have noticed that the ambient temperature is 110°F and intake temp 140°F and been unsurprised to see 6° correction. Why is this and how does the DME decide what the timing should be and whether to pull 0°, 3° or 12°?