Rounding Account

We have created an AR transaction and applied a credit memo to it, the invoice belonged to Canada Business unit, but the currency was used was US dollars the transaction also consists of a tax line as well, the conversion rate that was used is "User", the issue arise when the accounting happen and moved to GL, upon analysis the wrong rounding account is being picked, what is the behavior of the rounding account where is it being picked up on what basis is it being picked up (Please let us know the navigation) how can it be changed.