If rate currency field is blank pull from a joined form a default rate

If rate currency field is blank pull from a joined form a default rate

We have a billing form that contains a project id.  We would like help in the following please. 
If the client rate is not entered in the billing form we would like to pull the rate from the project id form from a billing rate field.  If the user changes the input it should not pull from the project id - billing rate field.