railing

My hand rail is secured to the plaster on the edge of my staircase at two points; As the staircase rises from floor 1 to floor 2, the metal railing screws into the ceiling of the first floor. The ceiling is plaster on metal lathe. At site one, the plaster where the old screw entered the...

Good at math but geometry eludes me - algebra, no issues; figuring out a mitre angle, and I'm on the floor in a fetial position crying :mad:! My go-to method has always been trial and error - little cuts until the joint works. But there has to be a better way?? Some, super secret, carpenter...