The required front yard setback depth of a lot in any residential district shall be modified where the average depth of at least two existing front yards on lots within 100 feet of the lot in question and within the same block front is greater than the minimum front yard setback required in the applicable zoning district. In such case, the front yard setback shall not be less than the average depth of such existing front yards on the two lots immediately adjoining or, in the case of a corner lot, the depth of the front yard on the lot immediately adjoining. However, that the depth of a front yard on any lot need not exceed 100 feet.