Variable Pitch Helix

Hi,

I am having trouble recreating this pitch on a feed screw.

It starts with two 6mm pitch revolutions, than goes to 12mm revolutions (3 total). I can get the helix to work  from 6mm to 12mm but from 12mm to 12mm always messes up the rest of it.  

Below is how i am trying to set it up, not sure what I am doing wrong.

SolidworksGeneral