I do a community centre, and 4 windows over a news agents next door, on the last Friday of the month.
On number 2 setting, I can do the two on a back pack fill.
Recently I have been using number 3 setting and I tried it today for this work.
Number 3 setting lets me work quicker using a higher flow rate.
I finished the job still having a quarter of a tank left, and in considerably less time. £24 for 26 mins work.
I used number 2 setting as most posts on here suggested, but number 3 setting can actually save you water by letting you do the job quicker.
It would be interesting if some of you backpack users gave it a try, and then let us know how you get on. Dai