There’s a simple new excel spreadsheet available on the Tools for Petrologists page. The tool Calculate fO2 Buffer allows you to determine the “delta” of a several oxygen fugacity buffers (for example: ΔNNO, ΔQFM, etc.) when the value of logfO2 is known. Here’s a screenshot of the inputs and outputs:
References: B. R. Frost in Mineralogical Society of America “Reviews in Mineralogy” Volume 25.