Anyone know how i set difficulty for my rigs?

I found some older information that seemed to suggest filling the password with d=x.xx would work but I can't seem to get it to lower now matter how low values I put in.

On my personal rig I tried 3 and 2.5 but can never get it lower than 5 (which means i get at best accepted shares once a minute)

On my mining rig it spikes crazily going form 17 down to as low as 11. Again setting diffulty seems to have no effect.

Any advice? Not possible to adjust?