User Defined Function - Sumproduct

Hi there

I have a very large formula that works. At the moment the formula exists in a cell on a worksheet and I have written some code to copy the formula to other cells on the worksheet and then after calculating the formula the code turns the resulting output into a value. I have had to do this as the formula is so resource intensive that if the formula lived in each of the cells the model would run too slowly.

I would now like to turn the formula into a UDF, so that I can protect it from the users, but I have no idea how to start... I'm not expecting a converted formula, just some hints on how to go about it ...

The formula is:

=IF(O$3="Opening Bal",O_Financials_Actual!O12,
IF(ROW()<ROW(modCashFlowStartRow),
(
SUMPRODUCT((I_Budget_Worksheet!$E$13:$E$288=$E12)*(I_Budget_Worksheet!$O$7:$BK$7=O$7)*(I_Budget_Worksheet!$O$13:$BK$288))-
SUMPRODUCT((I_Budget_Worksheet!$F$13:$F$288=$E12)*(I_Budget_Worksheet!$O$7:$BK$7=O$7)*(I_Budget_Worksheet!$O$13:$BK$288))
*
IF(O_Financials_Budget!$F12="Cr",-1,1))+
IF(O_Financials_Budget!$G12="***",O_Financials_Budget!N12,0),
SUMPRODUCT((I_Budget_Worksheet!$G$13:$G$288=$E12)*(I_Budget_Worksheet!$E$13:$E$288=Bank_Account_Identifier)*(I_Budget_Worksheet!$O$7:$BK$7=O$7)*(I_Budget_Worksheet!$O$13:$BK$288))-
SUMPRODUCT((I_Budget_Worksheet!$G$13:$G$288=$E12)*(I_Budget_Worksheet!$F$13:$F$288=Bank_Account_Identifier)*(I_Budget_Worksheet!$O$7:$BK$7=O$7)*(I_Budget_Worksheet!$O$13:$BK$288))
))

Thanks in advance
Peter