I assume, and for anyone that might wonder what the solution probably was, is the decimal digits is a bit of a red herring, i.e. you don't need to convert the string to a bunch of decimal digits.

Once you have the byte array, you just add the bytes together (i.e. loop through the array and add them to a Long type for instance), and then just do a Mod 100 on the resulting Long value, and use the result (not converted to hex).