Evidence of decreasing magnitude in the bearing capacity factor, $N_{gamma}$, in shallow foundations requires a different strategy rather than taking a constant soil friction angle conventionally used in computation of this factor. Because of the influence of stress level on soil friction angle, a more complex analysis may be required to compute the values of $N_{gamma}$ since the mobilized soil friction angle at failure differs from point to point beneath shallow foundations corresponding to the experienced level of stress. This can be performed by taking a variable field of mobilized soil maximum friction angle which can be achieved by the aid of the Zero Extension Lines (ZEL) equations. It has been realized by the authors in developing design charts in which, variations of $N_{gamma}$ with foundation size have been presented by the stress level based ZEL method. Following previous attempts, this research is devoted to extending these charts to circular foundations and furthermore, to provide an in depth investigation of the possibilities and advantages of using these charts. A comprehensive foundation load test database has been collected and recompiled for this research to compare the predictions made by the ZEL method in the suggested design charts with field data. The predictions by these design charts show a reasonable coverage over the entire range of selected case studies presented in this research.