its a while since I exercised these mental muscles!
I have tried to solve Z in the image below based on the most simplistic trig I can remember but keep finding that I don't have enough information.
(note the pic has been cropped and not to scale anymore - only showing the working part of the SRA3 page I describe in the problem)
The problem is essentially this:
Take a piece of paper 320mm x 450mm (SRA3) and fold one (right angled corner) to the exact centre (160,225) of the sheet and crease it. The crease becomes "Z" in the image below.
I am not concerned about the ratio of the short edge to the long edge in this instance (I understand the relationship within the A series papers) but rather want to know if there is enough information to solve for Z knowing only 160 and 225mm...