Select a specific number of faces from a known percentage in mode random

Hi All man :)

I have create this script: (Create a plane with more subdivision and select it)

try (destroydialog t) catch()

Percentage = 100.0 ---define the initial percentage 



rollout t "Test"
(
	spinner sp "Rate = " range:[0,100,100] type:#integer
	button b "Go!"
	
	on sp changed val do Percentage = val
	
	on b pressed do
	(
		obj = $ ---the object selected
		converttopoly obj ---convert the object in editpoly
		getFaces = obj.numfaces --get the number of the face 
		
		---Calculate the number of face to selected by specified percentage 
		numFaces = (Percentage / 100.0 * getFaces) as integer
	
		---create a array for all faces  by specified percentage value
		numFaces_array = #()
		
		for o=1 to numFaces do 
        ( 
        append numFaces_array o 
        ) 		


		subobjectLevel = 4
		polyop.setFaceSelection  obj  numFaces_array --select the face 
		

		print numFaces
		----
		update obj
	)
	
	
	
	
)
createdialog t

As you can see, the script determines a percentage of a known number of faces that are selected consecutively (1,2,3, etc.)on the subdivision plane.

How do random known number of faces across the whole subdivision plane?

Some days I feel, but I can not understand...

Thanks in advance :)