While I can attest that operating a contemporary jet is not an exact science, certainly there is hope that the engineering would be. PPM stands for 1 x 10^(-6), no units given - it's a simple multiplier.
That does not taste right. Perhaps mg/kg or ml/m^3, anyone?
A case of a point for the voices saying ppm is hard to grasp on the line:
- density of jet fuel 0,785
- density of Kathon 1,05
Which are the actual correct units to be used for diluting the additive according to AMM and the spec-sheet? Molecular, mass, or volume?