You have the 0.9v/cell part right, 5 cell pack - 0.9 x 5 = 4.5v, 6 cell pack - 0.9 x 6 = 5.4v, 7 cell pack - 0.9 x 7 = 6.3v, 8 cell pack - 0.9 x 8 = 7.2v.....



Its 0.9v/cell multiplied by the number of cells...