If you have a customer that has a spec defined, then you need to change the option to 'Use selected spec' in order to override. You cannot leave the option as 'Use Customer spec' and get anything other than the customer's defined spec.

After doing the override with 'use selected spec', you can then set the option back to 'use customer spec' and print with 'Screen' as the destination. This will re-save the option back to 'use customer spec'.

The options used when printing is what gets saved for the next time, rather than a 'default spec printing' option that can be set somewhere.