How do you calculate %xHKO?

Function doesn't work at all.

 Spoiler

Code:

(defun prob-hits (&key L H x n)
  (/ (loop for k to n
      sum (* (ncr n k)
             (expt -1 k)
             (ncr (+ (* n (- 1 L)) (* k (+ 1 H (- L))) x -1)
                  (1- n))))
     (expt (- H L -1) n)))

Oh yes, it's also apparently the probability to hit exactly the hp required (not including anything above it).