This info is from the Yamaha Rigging guide for 2011 It probably covers many model years
The upper mount bolt is usually installed to the 2nd hole from top.
MOUNTING THE OUTBOARD
MOTOR
2. Adjust the height of outboard motor so that
the anti-cavitation plate is positioned to the
boat transom bottom, or lowered within 25
mm (1 in.).
For planing boats, the anti-cavitation plate
should be positioned to the boat transom
bottom or slightly higher.
Note that unless you have data on current performance, handling, on plane time etc, as relating to published performance data, you may not be able to easily judge if a change is needed and if any change you make is an improvement.
The speed info usually comes from the pitot tube, that small hole on the leading edge of the lower unit. Depending on rigging, the water can feed all the way to the speedo as a fluid or there can be a pressure sensor that converts pressure to an electrical signal that ends up at the helm as a CAN bus signal or an electrical voltage if you have an analog speedo. It is common for the pitot tube and/or 'plumbing' to get clogged. A stiff wire may clear it up
