For the average driveway I run about 3 gallons of straight 12% bleach and about 16 ounces of sodium hydroxide w/surfactant through my 20% General Hi-draw chemical injector.
Comes out of the wand about 2%. After soaking the driveway thoroughly, I turn the machine off, take the injector off, and use a Maxima 36" surface cleaner with a 20hp 8gpm + 13hp 5gpm w/ a high pressure T to push 13gpm 3500psi total.
It is like mowing your grass it's so fast. Comes out much cleaner and faster with chemical. You don't have to double back or move super slow, you can actually walk behind it.