Obviously you want co2 as high as possible, but lower output burners often have less co2. Be careful with them, especially the older burners for some reason, like the Riello 40. Settings for them years ago, on local manufacturers spec, was 10.5 co2 for small boiler. That is correct, & although it can be a bit better, you will find sometimes smoke number above 0 a little. That is why I say go by smoke gun. Often better not to set too fine, & you will still have good readings. Use old spec of boilers, if you can get them, for settings, but you can only do your best & you want the boiler to be fairly clean running & not so close to running sooty.