• All Rates will be expressed in terms of multiplication.
• you will never do division on rates to translate currency
• so if you have 5.00 us and 
• you wanted to convert to CAD 
• you would never take the CAD to usd rate and invert 
• you would just find the usd to cad rate
• when you save a rate its reciprocal pair will be saved
• usd to CAD 1.28 will save the reciprocal rate .7785 will be save
• you will need to setup both currency pairs if you want convert bi directional currency transactions

When you create a  currency rate the inversion of the rate is added if the reciprocal pair exists.  So you can not have different rates for the same currency pair in the opposite direction.

Based upon external discussions and empirical data our current position is not to allow the reciprocal rates for the same currency pair to differ.

We have also tested this with actual closing prices for currencies  in numerous test cases and believe this is the appropriate stance.

Test on closing prices - real time prices will be impossible to test - constantly changing.