|almost always imperfect ceiling and wall joins.|
Take the piece and put in place ......
|push tightly in place|
Transfer the mark to the top edge and put the wood in the mitre block with the mark lined up very carefully with the right angle guide.
Put the saw through the front and back slits, making sure it is as vertical as you can and saw through making one (near) perfect right angled cut.
|one neatened join|