Hi,
The number of liters does not really matter if you are dosing the liquid according to the bottle suggested amounts.
At the end of the day, Excel and other Gluteraldehyde products does not have anywhere near the effectiveness of gas.
Unfortunately, no one knows what the actual numbers in percentages are. It would be very difficult to give a number as there are so many variables in the tank as well as there being parameters that we cannot measure.
The best advice is to not use as much light intensity with Gluteraldehyde as you would with gas.
Cheers,