Measure of environmental impact on phenotype
In between a long comment and an answer...

For a review of the theory behind the causal variables of phenotype, one might want to have a look at Biology Why is a heritability coefficient not an index of how “genetic” something is?.

One can investigate the impact of a specific environmental variable onto the phenotypic variance but the environmental as a whole is typically computed as whatever is left when other sources of phenotypic variance have been explained. As such you might want to read about estimations methods for heritability and maybe esp. the post How do heredity and regression to the mean work with respect to intelligence?.