On average if the system is set up correctly (in my opinion)one man uses 1 litre a minute so 60 litres an hour, say you actually work cleaning windows about 6 hours a day allowing for travelling in between then a one man set up needs to be 400 litres.
It would be better to get a larger set up if you want to grow the business and not go to the expense of buying another tank in the near future.
When I switched to wfp my first system was 600 litres as their was no one to get advice from, two years latter I built the next one with an 1100 litre tank and now 11 years latter we use 1200 litres per van - two tanks one of 800 litre with a linked back up tank of 400 litres.
I started off with a trolley system using 200 odd litres a day,.. but as you get WFP specific work coming in, big "Glass wall" commercial stuff etc you will use more.
Also,.. don't plan for what you'll use on your "average" day,.. plan for what you'll use on your "busiest" day. On Monday I worked 11 hours and used over 800 litres on a big commercial job,... it was a huge pain in the butt filling and loading the extra water in barrels into the van, and then pouring the water into the tank at lunch time,.. I was worn out enough already without that extra work!!