Hi case777,
I'm not familiar with the Turtle rules but, if the n value and ATR are one of the same, then most charting packages allow you to add ATR which will provide you with a numerical reading and, therefore, your n value. If you're spread betting and using a company that uses IT Finance charts then, unfortunately, you're out of luck as they don't offer the ATR indicator. If you want to know more about ATR, a Google search will reveal lots and, additionally, there are some good articles about it here on T2W in the 'Articles' section of the site.
Tim.