The stable isotopes (15)N and (13)C can be used to investigate the nutritional mode of terrestrial orchids and pyroloids (Monotropoideae, Ericaceae). Some of these plants are putatively autotrophic but meet their nitrogen and carbon demands by gaining organic compounds (e.g. amino acids) from mycorrhizal fungi. This so-called partially myco-heterotrophic nutrition is reflected by their isotope signature. The application of a two-source linear mixing model on delta values of such plants allows calculating the percentage of N and C derived from their associated mycorrhizal fungi. Here we present an approach to improve estimates of the plants' degree of myco-heterotrophy. Due to the presented conversion of delta values into enrichment factors (epsilon), results obtain a better resolution and data from various studies become normalised which facilitates combined representations and meta-analyses.