apply formula at result level

Hi,

I need to build a report to get a KF of inventory turnover, the formular used is:

<i>turnover =total consumption / average stock</i>

while we use the formular below to get the

<b>average stcok =(begin of stock + n stock at month's end)/(n+1)</b>

what I need is to based on user's date input(let's say 09/2005 to 11/2005 for example),

1,get the "total consumption" for Oct and Nov,

2,and also get

"average stock" = (Stock at Sep + Stock at Oct + Stock at Nov)/(2+1)

3, get the turnover = "total consumption" /"average stock"

let's say KF "Total cnsumption" for 10/2005 is 10, for 11/2005 is 20. so <b>"Total cnsumption" for Oct and NOv</b> will be 10+20=30;

and if KF "Stock at Sep" = 12, "Stock at Oct" is 24,

"Stock at Nov" = 30

then <b>"average stck"= (122430)/(2+1) = 22</b>so the "turnover" will be

<b>"Total cnsumption"/"average stck"= 30/22= 1.36</b>

I am not sure how the report lay out will be, but as long as i can get the turnover based on teh user input's month during, that would be good enough. so probably the report will look like:

"Month" "Totalcnsump" "Stck" "averageStck" "TurnOver"

09/2005 12

10/2005 10 24

11/2005 20 30

<b>result: 30 66 22 1.36</b>

Does any one have any idea how to make it done???

Your input is very much apprecieated,

ping