Thank you!!! OK I believe I have it working now!!!
I gave the ListLookup control a Named Control of ProjectLookup, keeping everything else as I had it originally.
I removed the Name from the Calculated Field, but kept it connected to the Major Project ID field
I updated the formula as below, using the ProjectLookup Named control, instead of the SelectedProject field.
On the form, when I select and project now, the calculated field is showing a result, and when I Submit the form, the ID (PO013) is showing in the Major Project ID field!
Thank you again so much for your help and being so patient with me!!!
It is much appreciated!!!