Density functional theory (DFT) studies have been undertaken to compute the magnetic exchange and to probe the origin of the magnetic interactions in two heteroand two homo-valent heptanuclear manganese disc-like clusters, of formula [MnII4MnIV3(tea)(teaH2)3(peolH)4] (1), [MnII4MnIII3F3(tea)(teaH)(teaH2)2(piv)4(Hpiv)(chp)3] (2), [MnII7(pppd)6(tea)(OH)3] (3) and [MnII7 (paa)6(OMe)6] (4) (teaH3 = triethanolamine, peolH4 = pentaerythritol, Hpiv= pivalic acid, Hchp = 6-chloro-2-hydroxypyridine, pppd = 1-phenyl-3-(2-pyridyl) propane-1,3-dione; paaH = N-(2-pyridinyl) acetoacetamide). DFT calculations yield J values, which reproduce the magnetic susceptibility data very well for all four complexes; these studies are also highlighting the likely ageing/stability problems in two of the complexes. It is found that the spin ground states, S, for complexes 1-4 are drastically different, varying from S = 29/2 to S = 1/2. These values are found to be controlled by the nature of the oxidation state of the metal ions and minor differences present in the structures. Extensive magneto-structural correlations are developed for the seven building unit dimers present in the complexes, with the correlations unlocking the reasons behind the differences in the magnetic properties observed. Independent of the oxidation state of the metal ions, the Mn-O-Mn/Mn-F-Mn angles are found to be the key parameters, which significantly influence the sign as well as the magnitude of the J values. The magneto-structural correlations developed here, have broad applicability and can be utilised to understand the magnetic properties of other Mn clusters.