In any "R" District, where the average depth of at least two (2) existing front yards on lots within one hundred (100) feet of the lot in question and within the same block front is less or greater than the least front yard depth prescribed elsewhere in this Zoning Ordinance, the required depth of the front yard on such lot shall be modified. In such case, this shall not be less than the average depth of said existing front yards, or the average depth of existing front yards of the two (2) lots immediately adjoining, or, in the case of a corner lot, the depth of the front yard on the lot immediately adjoining; provided, however, that the depth of a front yard on any lot shall be at least ten (10) feet including all projections but not required to be more than fifty (50) feet.