2SThe valuer shall calculate an average price per ewe, and an average price per lamb, for the regular cast ewes and lambs sold as aforesaid for each of the 3 preceding years. In calculating the average price for any year the valuer shall disregard such number of regular cast ewes and lambs so sold in that year, being the ewes or lambs sold at the lowest prices, as bears the same proportion to the total number of regular cast ewes or lambs so sold in that year as the number of shotts as determined bears to the total number of ewes or lambs in the stock under valuation.