Welcome to the forums and the Tableau Community, honestly using parameters in calculations two weeks in is pretty good progress. You nearly had the solution and it was not your formula that was off in fact you were so close!
Tableau does exactly what you tell it today... even when we sadly make mistakes (even silly ones). Lets look at the Marks Card. Currently we have a lot of things going on here.
We are actually using SUM(Revenue) as our text marker (See the ABC 123?). We want to make our Measure Values the text marker. Change this by dragging Measure Values onto the Text Shelf. This will then apply the measure values below SUM(Revenue) and SUM(Revenue Forecast) based on the Measure Names Revenue and Revenue Forecast to the correct positions!
The tooltip (that little box that comes up when you hover) was working because SUM(Revenue Forecast) is included on the Marks shelf so it is either on the detail or the tooltip. So in fact the formula was doing what it was meant to do!
This would be the corrected version of the Marks card.
We keep SUM(Revenue Forecast) and SUM(Revenue ) as details. If we do not do this and we hover over the chart in a Revenue area the tooltip will only show Revenue, it will not show the Revenue Forecast.
Hopefully this answered your question and alleviated your fears. Welcome to the community once more and if you have any more questions let me know.
Thank you so much for your thoughtful and in-depth response! What a silly error haha.