Custom Javascript Code for Acrobat 9 Form • Volume Discounts

Hello,
 
I'm an acrobat newbie working on an order form that needs some Javascript programming.
 
http://bootiebabe.com/order_form_npc01.pdf
 
I have the basic calculations set up on the form but want to offer a volume discount:
 
1,200 - 4,800 bottles: 10% discount
4,801 - 9,600 bottles: 15% discount
9,601 -12,000 bottles: 20% discount
12,000+ bottles: 25% discount
 
so it's just a matter of seeing what's the total quantity field and determining the discount percentage then calculating the Grand Total.
 
In the discount field I want to just show the percentage (10-25%), not the discount in currency...
 
It would also be nice not to have the fields automatically placing "$0" in all the fields (someone may want to print out the form and fill it out with a pen).
 
Any help would be most appreciated.