05ab1e explanation

    g<°              # 10 to the power of: (Length of the input minus 1)
                     #  i.e. 601 → 10**2 → 100
       ©             # Store this value in the register (`b`)
        ÷            # Integer-divide the input by this value
                     #  i.e. 601 and 100 → 6
         ¹®%9*       # Input modulo-`b` multiplied by 9
                     #  i.e. 601 and 100 → 600%100*9 → 9
              ®      # Push `b` to the stack again
               O     # Sum the stack of these three values
                     #  i.e. 6 and 9 and 100 → 115
                <    # And decrease this result by 1
                     #  i.e. 115 - 1 → 114