This contribution extends a previous work where the concept of piston angular anisoplanatism was introduced and the issue of sky-coverage for large ground-based optical interferometers was raised. We obtain expressions for the temporal power spectra of phase fluctuations caused by differential piston and photon noise. This allows us to evaluate the residual phase variance left in a fringe tracking servo-loop system and obtain estimates for the sky- coverage due exclusively to differential piston effects, concluding that in V-band the percentage of sky that can be observed is a few percent while in K-band we should be able to cover the entire sky.