I think you're picture is very good .. very clear! In any case, Sheet 021a Drywell provides your answer (below). The 30-1/8" shown is a generic distance that accommodates all motors and would be the distance you need at the top. Applying a little trigonometry and measurements from Sheets 008a and 008b (transom loftings), you get your question mark distance as about 4- inches less than the 30-1/8". Your question mark distance is about 26". This assumes the top of your forward transom is rounded as in my original no-cutout transom layout. You can probably shorten the bracket length (question mark dimension) to 24" if your forward transom has a flat top - what I would recommend. If you have a motor selected, then get the tech drawings from the manufacturer, likely available online, and they'll show trim/tilt angles and dimensions that will help you refine the answer further.