Correct. "After" can be ambiguous, but water exiting should hit the new slip 45 before hitting the existing 45 on its way out of the house.
For visualization, maybe remove the trap arm from that extender (piece with the yellow line drawn). Attach the tee under your drain and connect the tube from the disposal. That lets you check that the elevation from the disposal is right. Put the trap input under the tee. Then rotate the trap to see at what points the trap arm direction can meet the extension at 45 degrees. There may be a range, because of the extra pivot in the trap. This is the beauty of the slip stuff. You don't have to get it right the first try, as you do with glue.
Cut the extension in maybe the middle of that range if there is a range.