Crystal Reports formula syntax comparing fields

Message

   After 8 years, I'm coming back to Crystal and learning all over again.  
I have a report where one field will depend on the results of another.  
The first field "ID_Type" is a formula that returns either the PO number or the Contract number.  Based on what that result is, the  formula "Exp Date" will return one of two database fields- either the PO_End_Date or the Contract_Exp_Date.

For ID_Type, I have the following syntax -
If instr({Contract_Prepay_Balance.SortType},"PO")>0 then
  formula="PO #"
else
  formula="Contract #"
end if

I think I can use this same syntax for the second "Exp Date", but not sure
If instr({Contract_Prepay_Balance.SortType},"PO")>0 then

  Exp_Date={Contract_Prepay_Balance.PO_End_Date}
else
  Exp_Date={Contract_Prepay_Balance.Contract_Exp_Date}
end if

Thanks!