I've had a look at the manual, skyplus. Not terribly helpful I found.
The primary is the main control for rate of burning .......... from roaring to tickover. Just as you might expect.
The secondary is mostly to do with the "Airwash" airflow that keeps the glass clean.
Both have an effect, but I would set the primary to the level of burn that you want. They are unclear about how the secondary affects burn rate. Open it right up, then close the primary down enough to keep your desired level.
I often find that companies have very knowledgeable people behind a phone. Do ring them and ask for technical help. Much better than my guesswork.