I have been through 700 litres in a day on my own but more typically it is about 400 litres. A lot can depend on the type of flow control you use. Also, bear in mind that the type of work you do now may be very different from the type of work you will be doing in a year. Personally, I wouldn't want to go for less than 500 litre tank and would prefer a 650 litre to make sure. If you have some kind of flow control such as varistream or similar you may get away with a smaller tank. There are other things to consider too like hose bore and jet sizes as these can affect your water usage. Using microbore rather than half inch hose can save water. Also, using smaller jets can save it - 2mm is enough IMO but some manufacturers use larger ones. Although such details may not seem important, over a day it can end up being quite a few litres saved.