In this paper, we study the Waldschmidt constant of a generalized fat point subscheme Z = m1p1 + · · · + mrpr of P 2 , where p1, · · · , pr are essentially distinct points on P 2 , satisfying the proximity inequalities. Furthermore, we prove its lower semi-continuity for r ≤ 8. Using this property, we also calculate the Waldschmidt constants of the fat point subschemes Z = p1 + · · · + p5 giving weak del Pezzo surfaces of degree 4.