Very valid points being made here by Simon and Spruce.
The new website calculator aims to take into consideration the GVW as well as the PAYLOAD by checking the DVLA database. However, you are quite right - its not just a case of maths. Talking to the guys on here who have experience of various system and vehicle setups also is very important.
Lets say a customer has a payload of 900kg and the van has three staff at all times.
Often customers forget that it is not just a case of simply subtracting the weight of your system from the payload. I.e It simply doesn't work that 900kg payload - 500kg tank weight = 400kg left over.
The guidance for payload (which can be found in the t&c's of most vehicle brochures) reminds you to remove the weight of any passengers and 10% of the fuel from your calculations. So two passengers - strapping young window cleaners each at 100kg each with a further 10kg of fuel allowance suddenly mean that your payload of 900kg, minus 500kg system, minus 210kg passengers and fuel allowance, you are now at 190kg. What about the reels, the filters, the poles, the gutter vacuum, the pressure washer... etc etc...? Suddenly, your looking at being overweight.
What about the vehicle itself? The design of the system? A standard upright 400 litre tank with is non-baffled or only structrally baffled, is going to cause far more stability issues than a flat 400 litre. This is regardless of payload or gvw. We will simply not fit certain systems in some of vehicles due to the suspension being very soft. The payload is fine, the GVW is fine, but the vehicle itself is not suitable for carrying a volatile load such as water.
Positioning of the tank is also something that is very important - where is the safest position? What position will cause the least wear and tear to the vehicle?
Our tanks are specificly designed to be a Cube design - which controls the water amongst the honeycomb baffles. No more traffic light sickness!