I found the centre of the plank (500mm) and then offered up the saw. I lined up the centre bolt with the centre mark and then marked out the extents of the blade mechanism. This material was then removed with a tenon saw. My jigsaw is in storage with most of my other shop tools but I think the tenon saw did a better job.
When the piece was removed I offered up the second top plank to mark out how much needed rebating off it to allow the blade to sit down. Finally the saw is in place