Try this in the columns.
Example: Col A = Desc, Column B is YTD (Base in Period/Base in YR). Column C is CALC. Put Base in the Period Code, Formula is B/PERIODS. So what this will do is in a calculation on the YTD figure. If you run the report for period 6, it would be YTD/6.
Hope that gets you started. Formatting, etc. will be required.
Warren