Override Line Item is a driver that indicates whether to take Initial Demand Forecast Value or Override Forecast Value and it is a manual input by end-user, not formula driven. Based on it the Final Forecast Value Line item will give the result
Write the formula in Final Forecast line item,
If Override is true, then take value from the Override Forecast Value line item else take it from the Initial Demand Forecast Value.
When building a formula you need to think which is the most possible occurrence of the condition: is it more likely to have overrides in the data or not? In this case overrides are not that frequent so you should switch your conditions starting with IF NOT Override? THEN .... ELSE ...
This is the best practice when building formulas so try it, it should work.