It's not
The Red line that I drew indicates where I would locate a mid cleat.
Close, but no cigar on that one. On this profile of my Hunter 310, the dimensions on my screen come out as shown :
Or on a 31' boat, the mid cleat was installed by Hunter at (260/419) X 31' = 19' from he bow. This is also, thoereticaly speaking, the horizontal measurement to the centre of gravity (CG) of the boat. The vertical position of the CG does not come into play. I also find this position works perfectly for hauling the boat abeam.
If you heave on a boat abeam in calm water and want it to come directly to you without rotating, you want to pull through the CG. You are moving sideways so slowly that the influence of the keel is minor. Strong winds, different story. Strong current, different story. Design to the average conditions.
All very interesting but calculating the CG of the boat would be rather difficult to say the least. Seeing as you have the boat in hand (so to speak) there's nothing like trying to temporarily locate the position of the cleat in real life.
- Get a dock line about the length of your boat and tie a single knot in it about a meter from the end.
- Find the heaviest guy you know (maybe even your mother-in-law). Or, to avoid a confrontation, maybe get two people.
- Stand the boat off of the dock or finger with you on the dock.
- Have the heavy person(s) press the knot against the toe rail, roughly about midship, and stand on or near the knot to ensure the knot doesn't slip over the toe rail when the line is pulled.
- Have the person on the boat heave the line to you on the dock.
- (You) heave on the line and see how the boat approaches the dock or finger.
- Keep repositioning the line fore and aft until the boat comes in parallel to the dock regardless of how hard you pull.
This is what you will get once you permanently install the cleat but without all the previous holes from installing it in the wrong place.