A surgical drape includes a base sheet of thin, flexible material. A process step is performed upon a first workpiece in a batch based upon a process target setting. Optical fiber ends are located at the front faces. Finally, an index is generated according to the REITs, the obtained coefficients, and the weights. In one embodiment, the closed-loop circuit includes a critical path replica for providing estimated frequencies of operation necessary for a critical path in the integrated circuit device.