easiest way I found to calculate the correct amount is to go to a converter such as:
http://preev.com/Type in 1USD and it chugs out the correct Bitcoin amount.
divide that number by the current rate of Nobl on
https://www.coinmarket.io/takes about a minute
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The site you mention is currently giving a BTC price of $893 while Bitstamp has it at $824.5