Yes you do need to experiment with your own boat/engine.
I found that with the smaller plate level with the keel (as above) there was a lot of splashback into the boat, no matter what I did with the engine trim.
A block of wood under the engine mounting cured it completely, although it only raised the engine less than 2cm.
So that's why my picture showed the engine slightly higher. But I have the Tohatsu 9.8 two stroke which is known, I am told, to suffer from splashback. I never needed a riser block with my previous engine.