Testing if particle is looking away from camera

[V737]

You can't sample falloff maps with a particle system.

You can use a Script operator to calculate the dot product between a surface normal and the camera direction, outputting particles if the dot product surpasses your desired threshold value.

Thank you for the reply, I didn't try using a script in Tyflow before, just looked at the operator, it seems it will do what I want, just have to figure out the code. 
thanks!