Greetings Steve:
In order to calculate volume in standing trees, you need to determine tree diameter at breast height (4.5 ft.) - this measurement is referred to as DBH (diameter at breast height). The next measurement you need is the number of merchantable logs in the tree - this is commonly done by calculating number of 16-ft logs (8-ft is a half log).
As an example, a 20" DBH black walnut with 1.5 logs (24-ft) would contain approximately 180 board feet using the Doyle Scale. On good black walnut sites, you can expect 3-4 inches of diameter growth over the next 10-12 years.
Please reference the following publications to assist you in this endeavor:
http://ohioline.osu.edu/for-fact/pdf/0035.pdf
http://ohioline.osu.edu/for-fact/pdf/0062.pdf
Best of luck!