It's not so much which hole but rather where the anticavitation plate lines up.
According to the general Yamaha rigging guides, the cavitation plate with the motor down and parallel to the transom, should be even with the bottom center edge of the keel or up to an inch higher. That would be a good starting point.
FYI, although the hole spacing on the motor bracket is known, the location of the holes in the transom may not be 'standard'. Where they were drilled depends on who drilled them and if they used the standard template tool or just kind of measured.
After the initial setting, the only way to know if mounting is correct is to observe the operation at speed on the water. That is often a bit tricky since the position of the lower unit changes when someone goes aft to look over the transom to see where the cav plate is. This is especially true for smaller/lighter hulls like yours. Of course the cav pate position also depends on load such as passengers, gas, gear and trim.
