Calculating Average Daily Cells. Maintain your daily sales in a sales tracking spreadsheet. Enter daily dates in the first column and corresponding daily sales in the second column. Use the average function to calculate the average daily sales for the period in question.For example, say that you have two months worth of sales data in cells B1 to B61. Type =Average(B1:B61) into the formula bar of a blank cell.The resulting number equals the average daily sales for the period.aintain your daily sales in a sales tracking spreadsheet. Enter daily dates in the first column and corresponding daily sales in the second column. Use the average function to calculate the average daily sales for the period in question. For example, say that you have two months worth of sales data in cells B1 to B61.