Is that the ecover truck wash?
If so 3/4 of a bottle of diet coke 500ml. I have just taken the top of the bottle and keep it in the van so when I need to fill up I use about 425ml Ecover Truck wash then 5 litres of water. You can use a stronger solution and I often do. But for all new cleans I use that ratio. It is safe to double that ratio, but best not to let it dry especially when its hot outside.