Did you tell Will what power you wanted to run the rifle at? Will could get the power you want with the spring and then you fine tune it with the power bands, you need to use the flat slip washers so the spring can completely revolve, one slip washer in the bottom of the piston with the top hat on top of that, then a thin power band on the top hat with the spring end on top of that, then a slip washer on the spring guide with another slim power band on that and then the spring on the guide with the spring end on the power band, you aim to make the spring uncoil without any contact with the piston or trigger block, so all the torque from the spring isn't transfered to the cylinder, personally I would aim for 10.5ftp, 11 max.
Pete