Macro for identifying text in reference cell and inserting a blank value in target cell

Hello. I hope somebody can lend some assistance and guidance with the correct VB coding because I have been going round in circles trying some of the basic codes on webinars/podcasts and they don't seem to work

I have a Reference Cell (H2:I2) in which there is a drop down list with two options: 1/. "SALES INVOICE" OR 2/. "QUOTATION". This has been set via a simple Data Validation process.

Cell I25 (the target cell) contains a simple formula in it "=(I22-I23+I24)*16%". It calculates Sales tax at 16% rate for the three rows above.
Row I22 = the subtotal of purchased products
Row I23 = any discounts to be deducted
Row I24 = Any freight costs to be added.

When the text "Sales Invoice" is selected in the Reference Cell (H2:I2) then all the formulas contained in each of the subsequent rows mentioned should automatically calculate various values.

However if the text "Quotation" is selected in the Reference Cell then I need the macro to bypass the formula in I25 AND simply place the text "-----" ie. five space bars without deleting the formula "=(I22-I23+I24)*16%".

Any ideas please - somebody - anybody out there who can shed some light. I really need some urgent assistance on this one.

Thanks.