There are holes on the mounting bracket that allow you to raise or lower the engine as needed. A 25 inch transom is a 25 inch transom....not 26" or 27"
Question: what if you made your transom 27" and then mounted your motor as low as it would go. Your anti cavitation plate is now 2 inches above the lowest point on your transom. So you go and run the boat and and find that you need to lower the engine. How are you going to do that?
So to answer your question I would make it 25 inches.
strick