Supporting fractional values for a unit converter function

I want to extend a unit conversion framework that accepts fractional 
inches by letting value be a Lisp rational (like 7/64) and then treating 
it as a normal number in the conversion.

To write (units 7/64 :inch :mm) rather than the cumbersome 
(units ((float 7) / (float 64)) :inch :mm)

(defun units (value from to)
  (cond
    ;; inches to cm
    ((and (eq from :inch) (eq to :cm))
       (* value 2.54))

    ;; cm to inches
    ((and (eq from :cm) (eq to :inch))
       (/ value 2.54))

    ;; catch invalid combinations
    (t nil)))