Ian
Although it is an excellent product, Kryston Droppem is wax based, which makes it partly buoyant. Therefore a 'blob' of the stuff is relatively large weight for weight - ie for every sinking particle of tungsten it holds there is a corresponding amount of buoyant material. At the moment it is not the best sinking putty on the market.
I know this because I designed and manufactured the extruder head that Uncle Dave uses to make the stuff.
Have you tried Kryston Heavy Metal Extra? Heavy metal holds a very large amount of tungsten so a much smaller blob is required to create the same weight as droppem.
PS Yes FFF are the dog's danglies