Yes, generally 3 1/2 - 4 gallons bleach to 5 gallon bucket. That takes the 8.25% down to 6.6% and my 10:1 downstreamer puts around .6-.7% on surface. Really bad areas may get more concentrate. Like I said, only a few dozen house washes for me right now a year, so justification in buying bulk SH to just sit around is not worth the price of it going bad. (Not to mention no room for it).
So I just go down the street the day before a job, buy $15 of bleach to getter done.
With your info, if my math is correct and you have a 20% injector which I assume is the same as a 5:1, you should have had around 1.1% hitting surface, should be aplenty.
Should be a formula out there to make this easier, plug in known numbers to get what your looking for. Tim you got one?