The minimum lot width of any lot shall be measured at the front yard setback line as required for the district in which it is located. In cases of irregularly shaped lots whose sides are not parallel, the street frontage shall not be less than the 75% of the minimum lot width required; provided, however, that the lot width as measured at the front yard setback line shall be no less than the minimum lot width, as specified in the Zoning Schedule, for the district in which the lot is contained.