ive got a 650 and when filling it probably cuts off about 600ish as its never full but i use roughly about 4-450 litres a day, funnily enough since i turned my flow rate up im using less and getting through work quicker.
my old van i had a 400 and used to run close to empty by end of day and doing con,gutter cleans i was always tough and go, its nice now knowing i can run higher and use more water without the worry of running out anymore.
if somone was buying a tank for 1 man though i'd suggest they get a 500 for a perfect balance.