Forum Discussion
Default Drop-Down Based on Cell Value
It looks like you have figured out the strengths and weaknesses of data validation dropdowns.
If you add an empty string "" to your data validation source, you won't need to turn error alert off when you apply the formula.
If you need to reapply the formula after the user has deleted it by making a data validation dropdown choice, then VBA is the way to go. I suggest adding a button to the worksheet to call the re-initialization macro. VBA is also the way to go if you want to prevent the user from making a selection in M7 or S7 until after they have made a selection in the preceding dropdown. This could be done by a Worksheet_Change event macro.
I realize that you don't know how to write the required VBA code, but it is pretty easy. If you post your workbook, I'd be glad to show you how it's done.