Maintenance choices: Good, Fast, Cheap.
You only get to pick one and the airline execs always seem to pick cheap, while pretending the other 2 will magically also happen....
I'm not convinced this necessarily applies here. Ratio calculations are notoriously easy to get wrong. Even the very best people make simple mistakes when dealing with parts per million (because it adds an extra step to a calculation) and this is likely exacerbated when working at night, tired, and under time pressure. There are some simple and obvious steps that can be taken to prevent this happening again:
1. Provide a worked example of the calculation in the maintenance manual.
2. Provide an electronic calculation in the maintenance manual (input contamination, biocide, fuel quantity and it spits out the required dose).
3. Supply the biocide in smaller containers. Even the person who's absolutely confident of their calculation will think again if they realise they have to log, open and pour (for example) 60 bottles of the stuff into the aircraft.