A methodology to calculate numeric values of primary indication values for a hypothetical taxon in relation to one or several hypothetical environmental factors in a given regions, is proposed. Primary indication numeric values correspond to the minimum tolerance (Tm) of any taxon to one or several environmental factors expressed by an environmental quality index. These values are obtained for each taxon basis of the fifth percentile of the normal adjusted distribution curve, using the abundance of each taxon in samples taken at ten intervals or classes in the environmental scale of each factor or group of factors on the corresponding quality index.