If you wanted to figure out the shank length knowing the countersink will be 82 degrees, first we would assume you're countersinking to around 1/2 panel thickness (the recommended depth). Panel thickness in the below drawing is labeled (T).
Once this information is collected, we would apply the below formula:
10 GA = .1345 Nominal thickness (T)
Shank length is therefore: .067 + .088 which yields .155. To ensure that the shank is flush to under-flush, reduce the .155 by .010, resulting in .145. You would use this information to select the appropriate part.