I believe Armstrong states that for a single engine, you mount the flat deck of the bracket the same distance up from the keel as your engine shaft, that is, 25" for 25" shaft. the actual mounting on the bracket is raised about 4" which gives you the vertical height for your setback. then, if you use twins, you would use 20" shaft twins. For a 30" single you would use 25" twins. In all cases, the actual platform is set at the height of the engine shaft.
and yes, even after you measure up from the keel for height, and down from the transom gunnel for level, stand back and look at your bracket. On mine it was about 1/2 to 3/4" out of level, even though the measurements were ok. I'm glad I took a moment to stand back and check before drilling holes all over my transom.
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