AudreyB is correct ... if you are keeping it in the same proportions, then the measurements would be awkward!
Adding the fourth outside strips would only bring the finished measurement to 11".
You could change the centre square measurement to make up the difference.
Another way would be to change the one inch strips to 1-1/2".
That would result in a 12" finished block.
................ What needs to be clarified is whether you are meaning 12-1/2" finished or unfinished?
AudreyB and myself are both referencing re 12" finished, meaning 12-1/2" unfinished.
If you are talking finished, then a different story ..................