Nigel
Diagram 20 indicates a distance of 508mm between the inside edge of each hole. The diagram explaining the “theory of of the slack rope hawse” indicates 510mm center to centre.
Your safest distance is the 508mm inside edge to inside edge of each hole.
I am sure the measurer would be quite tolerant of a few mm difference…it is not a speed issue but one of stength of the thwart.
Barry Eastgate
National Treasurer/Secretary