Excel Formula for Scheduling Calculations

I am working on creating a work schedule for our agency using Excel.  I need to figure out how to assign a numeric value to letters (A=12hour shift) and how to assign a numeric value to another number (1=8hour shift).  it would also be nice to figure out how hours can be totaled if  military time elements are entered into a specific day (Ex:  employee works from 0700-1500 on a specific day;  distinguish those hours and calculate the sum for the week).  any suggestions?

thanks