Cosmology contributes a good deal to the investigation of variation of fundamental physical constants. High resolution data is available and allows for detailed analysis over cosmological distances and a multitude of methods were developed. The raised demand for precision requires a deep understanding of the limiting errors involved. The achievable accuracy is under debate and current observing proposals max out the capabilities of today's technology. The question for self-consistency in data analysis and effective techniques to handle unknown systematic errors is of increasing importance. This work is motivated by numerous findings of different groups that partially are in disagreement witch each other. A large part of these discrepancies reflects the different methods of handling systematic errors. Evidently systematics are not yet under control or fully understood. We try to emphasize the importance to take these errors, namely calibration issues, into account and put forward some measures adapted to the problem. Alternative approaches for some of the steps involved are introduced.