I have, by accident, gone out with a tank at 30ppm, I did a day of top quality estate work with it, approx 30 houses.
When I realised what had happened I expected a day putting it all right but never received a single complaint, I even spoke to a couple of the customers that I know well and they hadn't noticed any difference.
You can easily get away with 10-15ppm if your cleaning technique is good, do not spend too much time worrying about these things, just clean correctly and earn the money.