We may have a little nomenclature confusion, but I'm going to assume N.S.P. stands for Net Suction Pressure.

If that is correct, then I think the best way I can advise you on this is to lead you to my Article entitled "Net Positive Suction Head," which you can find on my Web site www.practicalpumping.com/articles.php.

That article clearly spells out the way to determine Net Positive Suction Head, and it does require you to have the atmospheric pressure included as long as that is the pressure that is on the free surface of the liquid in the tower.