Marc,
I am not positive that @Elvic_OngGX answered what you were asking. My interpretation of your question, is that you want a single "r" value as in a Pearson correlation, that represents a Multiple Linear Regression. If my interpretation is correct, the answer is your R Square or adjusted R Square is that value. It represents the amount of relationship accounted for in your regression. Contrary to the "r" in Peason, where the positive or negative sign denotes the direction of the relationship, the squared "R" value of a Multiple Linear Regression may have both positive and negative terms in the equasion(see the beta coefficients), therefore, the squared "R" is your representation of the correlation.
Jim