When you buy those solenoids seperate they are normally sold with hose connectors at both sides. Meaning they are made to fit inline. Some people remove the input hosetail and screw it into the regulator. I have mine inline.
That seems about right for your 400L tank. I go through a 2kg FE every 2 months or so on my 250L tank so it seems about right based on this. Of course every tank is different and a lot depends on your injection method and its efficiency, how much is gassed off at the surface and how large your plant mass is (the more plants the higher the demand).
From what you are saying 5kg=100 days on a 400 litre tank, therefore 50g per day or 0.125grames/litre per day.
compared to 2kg lasting aproximatly 60 days on my 250 litre tank, therefore 33.33g per day or 0.13grammes/litre per day.
I find it best to concentrate your efforts in sourcing cheap co2 (my best buy so far is 9kg of co2 for Â£5.50 (second hand extinguishers found on on ebay)) rather than turning the surface aggitation and/or total disolved co2 down as low as you can. Just watch things closly when you near the end of a bottle as a co2 bottle dump can cause an unwanted spike in the total ppm disolved bringing it up to potentially dangerous levels for your fish (commonly called gassing). I now swap out a bottle as soon as the high pressure gauge falls below 45bar purely as a precaution as some of my fish are very precious to me.