A 45 would send it into the ceiling area and then we might as well use a vertical pipe out the roof. The problem we face with a vertical install of a chimney is that the addition used to be a carport years ago in the 60's in front of the mobile home. The residents converted the carport into an addition by putting a roof over the steel roof and enclosing the carport. They weren't the brightest people in the county. It is not as bad looking as it sounds, just a construction nightmare.
In photo The green area on chimney is approx location of where the horizontal pipe connects to chimney.
The blue line is indicating the hanging ceiling of family room made of fiberglass panels.
The red line obstructed by trees and chimney indicates a steel roof between the actual roof and ceiling panels, there is also 20+ inches of insulation below the steel roof.
Not to mention any other hidden surprises we may find in this crazy addition that was done
So that is one reason we prefer to use the original chimney.