The naturally occurring layered quasi-2D compound Ca2.5Sr0.5GaMn2O8 with mixed-valence of Mn3+ and Mn4+ (Mn3+: Mn4+:: 1:1) has attracted a considerable attention from the low-dimensional magnetic ordering point of view. We propose to investigate the nature and dimensionality of the short-range magnetic ordering both above and below the Neel temperature (over 140-170 K) in the electron doped naturally occurring brownmillerite type bilayered compounds,Ca2.5-xLaxSr0.5GaMn2O8 (x = 0.05, and 0.1). The evolution of the short-range correlation with increasing electron doping (x = 0, 0.05, and 0.1) will also be brought out. The diffuse scattering instrument D7 will be employed in its polarization analysis (elastic) spin-flip mode to filter out the weak magnetic diffuse scattering from the total scattering. The evolution of the short-range magnetic correlation length across the magnetic phase transition will be revealed. A magnetic phase diagram in the x-T plane will be drawn. The expected outcome will have significant impact to understand the physics of low dimensional magnetism.