I don't suggest trying your procedure. You will not be able to see, or "wish" that final piece into place, and the open time of the glue will work against you.
Take very careful measurements of the layout. Assemble and install in place the entire W/O assembly and trap ( on the building pipes, not to the tub.) Dry fit the tub in and out a few times to make sure all lines up. Finally drop the tub in and assemble the waste flange and the overflow flange.
Be careful trying to dryfit PVC. You cannot really assemble PVC fittings dry to the final depth they will go to when assembled with glue.