Exchange rate in F110

Hi,

How can we maintain exchange rate for different currencies to be used in F110.

For ex: SGD from USD

For example if the Current M rate for SGD to USD is 1.56 , but the user want to execute F110 and pay a pre determined rate of 1.60.

Kindly suggest where can we maintain this

What is the standard procedure for maintaining this type of requirement

Thanks

SRK