Mitre joint on church pew

[macduff1964]

I have a pair of oak church pew ends that I want to put in the corner of my dining room and mitre them together.
The seat bases are horizontal, so that mitre is easy :D
But of course the seat back is angled slightly back from the vertical [not too far - you wouldn't want the congregation nodding off....] so I'm baffled as to how to calculate the 'off vertical' angle to make the other 45° cut.
I've trialled and errored :? and i'm now running out of spare end offcut.
Anyone out there with the magic number?
Macduff1964.......

[goose]

What angle is the seat back from vertical?