help with PH order

I'm trying to figure out how much PH to order (I know this is like the 3rd post in the past couple of days about this).  I want the hair around bra length, and I'm having them pinchbraided in, so I guess I could cut the PH in half.  I don't wear my hair really thick, it's probably what would be considered average density.  I think when I used hh it was about a bag and a half maybe of 18" hair.  So how much would i need?  Thanks so much!

Thanks guys!  I'm not doing visible braids, they will start at about 2" down from the crown, and about 1" from the hairline.  You're right Jenny, I guess doubling them makes a lot more sense than cutting them.  Like I mentioned, I don't wear my hair very thick, it ends up looking more like the thickness of the average person's head of hair, which is multiple steps up from my thin horribleness!