When using the CO2 the plants are able to uptake the nutes in a more rapid way, basically, it is like steraroyds, when you take them, you would need to eat more protein and carbs to keep up with the new demand for muscle building blocks. Same with the plants, the CO2 acs as a catalyst which allows the plant to grow at an accelerated rate, but needs more food to keep up with the demand.
As far as the type of increase, you should just bump it up and listen to them IMO.
Adieu