If I understand well what you trying to do:
Increase the CRO of the cell that you want as serving cell in the desired area(leave the PT=0) Switch the cell to a higher layer and increase the layerthr. The first change assure you that the comunication begins in the desired cell and the second that the comunication continues in that cell
You need to be carefull and try with small steps first
Also check if in that area exists a border of LAC border