You might be suprised with how much water you use on a quicker flow. If you have water storage at home ie, ibc tank and you work close to home. Give it a try on a higher flow rate and see. If you do run out, nip home for a top up.
I have 1000ltr ibc at home and a 500ltr tank in van. Takes about 5 mins to fill up from empty. But thats regardless, as i never run out per day. I use flow of 70 because i use 4 jets and it gives an even spread of good flow. On a first clean i go up to 80. But on a normal day, i would probably use 300 - 350 ltrs of water on a flow of 70. As seen on previous threads, i use a valve tap, by my waist on a holster to stop water flow between windows etc. Never ran out (unless i forget to top my tank up before i set off....done that once b4)