Front yard depth. If there shall be a dwelling or dwellings erected on the same side of the street within 300 feet of either side line of a lot, the front yard depth of such lot shall be not less than the average depth of the front yards of all such dwellings within said distance, and the required front yard depth shall be not less than the average depth of the front yards of any dwellings on the opposite side of the street within a distance of 300 feet of either side lot line, extended to the opposite side of said street. In the event that there are no existing dwellings within 300 feet on either side of the street, the front yard depth shall be not less than 40 feet. In no event shall a front yard depth of more than 100 feet be required.