Tohatsu gives worst spalsh back issues compared to other brands engines like Yam, it's a tail design, what stated on owners manual is just a refference from where to start but can vary depending on the boat's hull shape.
The only way to know how high you need to raise the transom is go for a wot spin with well ballanced boat as to have boat near paralell to water level and have a light spotter check how high water flow is passing above small upper plate, that's the distance you need to raise transom for engine to perform as expected.
The tech explanation is very simple, under upper deflector plate there's a triangular area that cuts perfectly water flow, over small plate, where the muffler is, the shape is squared, no water flow is being cut, that's the horrible splash area.
My rib/Tohatsu engine had same problem now that engine has been perfectly height tunned the overall handling, speed and performance is oustanding.
In this example the 30 HP engine was sitting 1" lower, was impossible to plane, excessive water splashed over transom. Wood shimmed the middle cut to have a straight transom and problem was solved forever. This fine tunning height experimentation must be done under trial and error and above all, boater must have a bullet proof patience to do it once correctly and forever..