I have been looking for a van, and wondered how you work out the minimum payload you need ?
For example if I want a 400 ltr tank with two workers, the minimum payload would be ?
What is the easiest formula to be able to work this out please ?
Thank you 
There isn't a fixed formula as it depends on what kit you want to carry.
The one that I personally use as a single operator (therefore less people weight) is the weight of the water plus 500 kg. I can stretch this to 400kg (i.e. an extra 100 litres of water) when I have warning of a large job. This enables me to run down the diesel, remove any kit that I won't need for that job, go on a crash diet etc
. Some people are more minimalist than myself regarding what kit they carry plus the chances are that they weigh less than me.