It seems like maybe this should be in a query instead of the report. Then you don't need any sort of stop gap due to time limitations. Create a table for MonthWeights and do it right. It will probably take less time than some hard coded messy case expression anyway.

create table MonthWeights

(

MonthNum int primary key,

WeightMultiplier numeric(5,2)

)

insert MonthWeights

select 1, 0.04 union all

select 2, 0.06 union all

select 3, 0.09 union all

select 4, 0.08 union all

select 5, 0.09 union all

select 6, 0.08 union all

select 7, 0.09 union all

select 8, 0.09 union all

select 9, 0.08 union all

select 10, 0.09 union all

select 11, 0.08 union all

select 12, 0.13

Now just join to it and use the value in your calculation.