1/100 th should be the proper scale factor indeed, as the base PopcornFX editor units are meters, and Unreal's are centimeters.
If you're setting this attribute from a blueprint, you don't have to scale it manually, there's a dedicated checkbox in the 'Set Attribute' function for when you're setting coordinates, see here:
the 'Apply Global Scale' checkbox.
What space are the coordinates passed in this float3 attribute in? worldspace ? relative to the effect instance ?
did you check you were using it in the popcornfx script in the correct space?
A spawn script's 'Eval' function works in localspace
A spawn script's 'PostEval' function works in worldspace
An evolve script inside a localspace evolver works in localspace
An evolve script not inside a localspace evolver works in worldspace
A layer script works in worldspace
Does this help?