Don’t mark the center tube (20 mm engine mount tube) for the fin lines yet!!!
Wait until after the motor mount is glued in place. This way you won’t have to worry about lining the engine nook up with a drawn pencil line. It seems you never have enough time to get a mount in place while the glue is setting up!
These two steps (6 and 8) should have been reversed. Glue the mount into the center tube up to the pencil line, then mark the tube using the engine clip to line up the center tube marking guide.
You can cut out the tube marking guides and mark the other tubes now.
I would recommend marking the names of the center line and fin locations on the tube with pencil for identification later on.
The wing and fin layout on this model is different. You'll find it easier to pre-mark all the locations for later gluing.