If you read the "Introduction" at the beginning of Appendix P, it states that this particular Appendix is only valid for tanks in excess of 120 ft (36M) in diameter. Therefore, if you perform these same (Appendix P) computations on tanks less than this limit, the results are "suspect".

PVP-1279 was developed specifically for tanks less than 120 ft (36M) in diameter. You should obtain different results when comparing the two methods.

When should you use PVP-1279? When your tank is outside the scope of Appendix P (i.e. smaller than 120 ft in diameter). As to your results, e-mail your input file (.tki) to techsupport@coade.com so we can verify things with your numbers.