You're correct about the 40 to 50 gal per roof. On about half of them in the ritzier sections it takes around 65, but you sometimes run into situations where you may have to hit a part of the roof twice. I'd rather have extra than not enough.
I also do pressure washing and concrete cleaning. Many times I get to a job and the HO asks me to clean their drive and or house. "Yes sir" I have the time and the chemicals to to that.
We are reasonably new and learn from every job. We started out with a 6 x 10 single axle trailer which I pull with a Tahoe. We have a 250 gal tank for water and put on a 100 gal tank for chlorine. We have 3 reels of hose, 300 ft. press wash line, 300 feet low press. hose, 200 feet water hose. We have a big press. cleaner on the back. If you have all tanks full it would be really too much at 8 lbs per gallon. Remember most places you use their water and if you mix a 3 to 1 sh your not bringing alot of weight. We usually bring extra sh. I will leave you with the thought, you can never go to big on your trailer, tandem axle,brakes,and an extra length and width are all nice to have. We need to expand, another trailer would probably cover the need but am looking at a Isuzu box truck,which for the moving bill board factor would be good. I will send you an email on our rig. We had a nice sign go across the back of which we have had calls and we typically stand it up at the jobs. We had decals made for the sides of the tanks and of course magnets for the doors.