With the Bittrex BTC-BCH pair how do I order enough BCH to exactly spend one BTC?

With the Bittrex BTC-BCH pair how do I order enough BCH to exactly spend one BTC?

Bittrex orders have their lot size/quantity in BCH and price in BTC. If I only have one BTC in my wallet how can I convert it all to BCH?

To ask another way: How do I know the exact quantity of BCH to order so that I fully convert my one BTC to BCH.

Note this question applies to other pairs and other exchanges. I am also looking for how fees would come into the equation as if I request too much BCH there might not be enough BCH in my wallet to cover fees.

http://bit.ly/2QQwJLB