The lot shall contain a minimum of six acres in area. However, the actual minimum area required shall be determined on the basis of the number of animals to be kept on the site; in a ratio of one acre per each animal to be kept. However, recognizing that it is conceivable that the application of such a straight line ratio may result in a minimum required area in excess of that actually required for the intended use, the board may, for good cause shown, reduce the effect of calculating lot size in applying the 1:1 ration on a straight line basis.