Most days I could scrape by with 400. There are days when I couldn't do it though. Depends what type of work really. I have no varistream fitted so having one might help. Also, low flow can slow you down.
Paul I would defo recommend you have one mate, high flow is good put there is a massive amount of waste, where you can control the wate without affeting speed, I monitered this over about 6 months and now, can do a days work with about 400L, but i have extra as nothing worse than running out, better to have to much than to little imo, I think you would save on water trying this mate honestly i do