Last text value in a range

Hello averyone, my dependant validation works as follows:

1. On Cell D1, I can choose an optional-value.
2. Depending on the value of Cell D1, I can choose an optional-value for E2
3. Depending on the value of Cell E2, I can choose an optional-value for F3.

So far so good.

Now, how do I make F4, F5, F6... to reference to the last non-empty cell in Column E.