hello all!

I have a spreadsheet which is an order form with products down the left, then columns where you can enter quantities required, then the line total is calculated on the far right. What i am trying to achieve is to limit the total value of items that can be added to a maximum value of 15.00GBP. So, the range E9 to I89 contain the cells where people can enter quantities. What i have tried so far is a data validation formula =MIN(K7,SUM(E89:I169)). K7 is a cell which contains the value 15.00. I have also tried changing the formula by locking the columns or the rows using the F4 key but that hasnt solved it. Please can someone help?? thanks...