Unable To Set Vnd Currency Rate


I m newbie and try to set up my store with 2 currencies. one is USD (primary) and the other is VND. The recent USD/VND rate: 1usd=21060VND and this mean 1VND=0.0000474834USD (xe.com). How can I set this rate in my VND currency in cs-cart?

I tried and I only can set after 0.xxxxx (up to 5 rounded)