Without wanting to blame the hapless engineer, Googling something and then using a random calculator found somewhere on the internet, but without fully understanding the terminology...........Why did he not have access to a proper procedure and a proper, validated calculator?
Engine critical procedures, such as opening the cowl flap locks, the engine oil caps etc., have mandated dual inspections. Why not for fuel additives? Fuel is pretty critical !
Engineers attending aircraft I have flown, have print-outs of the procedures, and often phone another authority if they are not familiar with a particular procedure, to check. And that is fine and sensible.
Why this guy did not phone his boss or another engineer to check his figures is beyond me.