On my 180l tank I get through about 0.01grams/litre/hour of CO2. So for me at 8hours and 180litre tank that is 180 * 8 * 0.01 -> 14grams per day. This gives a green/yellow drop checker "everywhere" in my tank. I also have over 5000litres/hour flow rate, 2100l/hr from two filters and 3200l/hr from a powerhead.
So, you will be looking at least 30grams/day CO2 for 340litres. Get a pair of digital scales (Argos £10 ?) weight your CO2 cylinder before CO2 on and weigh is afterwards. 30gr should be easily measurable.
Your filter(s) flow rate will need to be at least 3400 litres/hour if you want to have any chance of succeeding with CO2. Flow and circulation are absolutely essential or else plants suffer as they are not being supplied with enough CO2. This is very hard and time consuming to get right.
Really, to help more, we need details of your tank dimensions, filters your are using, lights you are using, ferts dosing scheme your are using. All these things are quite critical in achieving success in a hi-tech CO2 injected tank.