I solved the 'measuring' problem by driving two temporary wood-screws into wall's plank. After marking and fitting cleats, sides and hooks to ramp, I was able to remove those two screws.
Down-side, fitting those cleats slightly twisted the ramp, such I had a fight engaging 'hinge' bolts.
Up-side, the ramp is now available for cat-prowls. Note rubber tap-washers as 'flexi-mounts'...
Cats being cats, Ms Bertie isn't the least interested, while Ms. Robbie sits on mid-wall shelf and just
studies this latest apparation...
May show more interest if local pigeons land and strut on the canopy...