Position and HistoricalTrade

Position and HistoricalTrade objects need account exchange rate property because sometimes when GrossProfit is zero it's not possible to calculate what was the exchange rate between the traded counter currency and the account currency.
I may trade USD/TRY and have an EUR account. What's the relation between TRY price changes to my account currency EUR?
I need it in money management calculations where I need to use percentage profits and commissions needed to be in counter currency which I need to convert from the account currency exchange rate...

Normally even it is possible to calculate an approximate exchange rate from the relation;
exchangeRate = position.GrossProfit / (|position.ClosedPrice (or current price) - position.EntryPrice| * position.Volume)
it lacks precision because the gross profit only has 2 decimal places and when gross profit is zero the net profit is negative due to commissions and then it's impossible to calculate the exchange rate which the net profit has been calculated.