The Short Open Load Reciprocal-Thru VNA calibration, abbreviated SOLR, is a valuable technique to use when a well-known Thru is not possible for the RF probing of two or more device ports. This occurs in cases such as double-sided device probing, rectangular port configuration or when the probes themselves do not match in tip configuration. In those cases, the Cascade Microtech WinCal XE™ software can be a life saver, enabling easy completion of a VNA calibration using poorly characterized Thru structures. In some instances the device under test itself can be used as the Thru most conveniently. But just how poorly can the Thru behave and still achieve an acceptable calibration? How much return loss, how much insertion loss can be tolerated? This discussion will answer those questions and provide guidance in choosing appropriate Thru structures for the SOLR wafer-level calibration.