I don’t understand what you are trying to do. In @Darren_Murphy’s example, he is just using the second template column to attach the letter ‘U’ to the number. He is not doing anything with any other table. You don’t even need the template column if you are just expecting a number.
If you follow his example exactly to get the tables set up, everything should work, and like I said the only modifications would be with the form itself.
For the first template column you need the User Row ID like you have from the user profile, but it seems that you are trying to get a sequence number out of it. It should be the user Row ID so you can use it to create the link between the user table row and the lookup table row.