[Ifeffit] k range

Dear all, I always appreciate the big useful help from everyone. I have a basic question. I think a similar question has been arose long time ago. (But I lost the mail..) My sample has very small amount (under 1 wt%) and cluster size (< 1nm). The spectrum is always noisy at high k range, so I use usually k=2 to 10 or 12. It depends on the noise of the spectrum. Of course, if I go further down, it is better. My question is how low can I set the k range? Is there any minimum range for reliable result? Please let me get out of this dilemma... Thank you always. Steven