Working with Expression Builder - Account Onwer mapping to Service Request Owner

I reviewed the Working with Expression Builder - including examples but wasn't able to make this work.

I have a an account record that the owner is a customer service rep.

When I create a Service Request(case), I want the service request owner to default to the owner on the account, not to the case creator.

Assignment rules are complicated as we do not have 'good logic' for our account ownership.

Any ideas would be helpful!

thanks,

Ann