There is a point of diminishing return for the standard HHO boosting. Where that is? varies case by case. different types of cells, different types of cars, ...
id start at 1/4 LPM per litter of displacement. measure gains. then try 1/2 LPM and so on. when and if you start going the wrong way you no were to stop.
As Far as to much damaging the engine this is rare. My mentor ruined 2 engines with Too much HHO. But what he was doing was waaay out of main stream boosting, not likely for most of us. I use a Exhaust Gas Temp Gauge for monitoring my engine for potential damage ranges.
When you're one step ahead of the crowd you're a genius.
When you're two steps ahead, you're a crackpot."