This is how I did it. I used a variable to set a "Root Folder" that needed to be removed from the path.

Then remove from the path:

A is:
replace(outputs('Get_item')?['body/{Path}'],variables('RootFolderName'),'')
B is simply:
substring(variables('DestinationFolderName'), 0, sub(length(variables('DestinationFolderName')),1))
Then use the Output:

You need to persist with the highlighted parts in this step - it might complain about syntax but just keep trying! e.g. you will need to type in the "Destination Folder" then add the "Dynamic Content" - it is not an "Expression"!
C is the Identifier from the "Get item" step
D is the ID from the "For a selected file" step