The ever increasing need for a representative statistical sample in empirical research has created the demand for an effective method of determining sample size. To address the existing gap, Krejcie & Morgan (1970) came up with a table for determining sample size for a given population for easy reference.
There is no need of using the formula since the table of determining sample size has all the provisions you require to arrive at your sample size.
Krejcie, R.V., & Morgan, D.W., (1970). Determining Sample Size for Research Activities. Educational and Psychological Measurement.
Small-Sample Techniques (1960). The NEA Research Bulletin, Vol. 38.