This functionality is currently available via administration panel.
On the Cross-rates tab you can set the adjustment to the currency exchange rates when they are taken from some single source (e.g. Central Bank or whatever). This is necessary in order not to loose on conversions.
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To set these cross-rates,
Go to the contract, tab cross-rates.
Click Edit to switch to editing mode.
Fill in required % for each currency (note that you can set different amount of adjustment for each of them).
Click Save.