thats a good site for measurements FL!!
I did the same as FL, We set up a platform got the bracket against the boat and marked to holes.
But sounds easier then it really is!! I guess it is easier if you have 3x3 individual washers on each bolt.
But I made a full bracket on the inside of the transom, there fore I had to get all the screws from the outside of bracket threw 1 hole then threw boat, then threw inside hole.
Was a PIA getting them all to match!! :-/ I thought this would be the strongest way!