How make this modeline item be recalculated more often?

[seberino]

   (setq default-mode-line-format (list ""
                                   'mode-line-buffer-identification
                                   "     "
                                   (system-name)
                                   "     "
                                   '(:eval (format "%s" (+ (string-to-int "%c") 
1))) 
                                   "Line %l Col %c"))

I've been playing with adding one to column number in modeline and
came up with the eval element above....

It evaluates to a 1 when column in zero but it is not updated
when I change the columns!?!?!? How make it be reevaluated
when %c changes???

CS