Setting your outboard up to the correct transom height.

Hi all

What is the effect of raising or lowering the outboard motor on the
transom? All outboard motors are built to suit 20"or 25" transom
heights etc, but if the motor is set slightly higher or lower what is
the effect on boat performance. I read a boat review where they
suggested lowering the outboard by ½" would improve performance. How
can you tell if your's set up perfectly as apposed to the motor
manufactures recommendations.

Bruce