Greeting forum.

I have just received my NN3 Rd16 rotator as a package, I have also just purchase an sigma 10-20mm f3.5. The camera I am using is canon 450 or xsi. Could someone help me set this up.. I have search the web and found out that the NNP of the lense is 6 mm behind the gold ring. However I have tried and tried but fail to get a perfect alignment using serveral technique to find the nordal point. I am max out at the top rail of the NN3 and I don't know what I could have gone wrong... Please help .

thanks in advance.