Kind of got a solution which is not quite what I wanted, but will do.

I have a textbox called txtMaxSales and in it, I have the following expression

=(math.Max(MAX(Fields!Sales.Value, "Data_lbPlanResult"),MAX(Fields!CompSales.Value,...

This might help explain in better,

SELECT

a.bondID,

a.rateID,

a.BondWks,

a.BondForecast,

x1= CASE WHEN a.bondWks >=1 THEN (a.bondForecast * wk1)/

CASE WHEN A.bondWks =1 THEN (wk1)

WHEN a.bondWks = 2 THEN (Wk1+Wk2)

WHEN...

Cheers that has seemed to do the trick