using case or if function

[river bend]

I have 4 fields defined as miles, rate, gross_amount, volume_amount and flat_rate. The gross_amount field needs to calculate this scenario: if miles has a value then miles * rate if flat_rate has a value then flat_rate if neither has a value then volume_amount (which is a calculation field) needs to return volume_amount * rate. I have tried different scenarios but not able to make it work. 

 

Case ( IsValid ( flat_rate ) ; flat_rate ; IsEmpty ( flat_rate ) ; (miles * rate) ; volume_amount * rate  )

[Fitch]

Case ( 
not IsEmpty( miles ) ; miles * rate ;
not IsEmpty( flat_rate ) ; flat_rate ; 
volume_amount * rate  )

[river bend]

works great. thanks