In this work, the lower and upper bounds for total A-electron energy (E) was studied. A method is presented, by means of which, starting with a lower bound E-L and an upper bound E-U for E, a sequence of auxiliary quantities E-0, E-1, E-2,... is computed, such that E-0 = E-L, E-0 < E-1 < E-2 <.... and E-infinity = E-U. Therefore, an integer k exists, such that E-k <= E < Ek+1. If the estimates E-L and E-U are of the McClelland type, then k is called the McClelland number. For almost all benzenoid hydrocarbons, k = 3.