Customer Data Base

Looking for a simple way to enter data on a template and have it relate to a letter. If I enter 458 in one column the next column should automatically read P Each number has a letter of reference. 1=M 2=P 3=S 4=P 5=S and so on...... There are total 2500 numbers 0 - 2500 and each number must relate to one of three letters M, P, S. Right now we have to manually enter each letter that relates to a number thanks