I have to determine the proper size of a storage space and I have 42 months of forecasted data. I ran the stats and have the average pallets received each month (assumption is the amount received will be shipped same month), my thought was I could just take the average times 3 stdev and here is the amount of space I need. The space will need to between XX and YY. (Average is 50 pallets with a stdev of 10, so 50 + 30 = 80 pallet spaces are needed)

However they now want to hold three months of inventory at all times, so my question is how do I now calculate the stats to get my sizing numbers.