i use mine at 12 oclock but if you are using a trolley (and scrims does look good) you will automatically find your own best setting per job because you will find you are running out of water sometimes.
A good bit of advice is to make sure you start at the furthest point away from your van and work towards it.
Also, if you are using a van get a reel of microbore, a water tank and submersible pump when you can as it will be quicker than faffing around with half empty barrels and you will have a van mount as well.
Another way to make your job easier and much quicker is to buy extra long pole hose. eg, 17ft xtel buy 45ft of pole hose. You can wrap the excess around the handle when not needed and you will be able to park the trolley in 1 or 2 places only and reach all the windows of the house. The pole hose that comes as standard will mean you will have to move the trolley every couple of windows. A right pain and time consuming.
Good luck with it.