Nobody flame me here, but generally speaking, the strut barrel is about the number of inches from the transom as the boat length in feet. However, a low degree strut will like to be a little farther forward. I would say go for 22-23 inches from the transom since you said your boat is a cruiser (21 or so feet). Don't measure from the cav plates, because they are different on every boat. Several other factors also affect this decision, such as horsepower, intended use, type of rudder, etc..... Also keep in mind that going from a 12deg strut (are you sure?) to a 7.5 is a HUGE change, and may require the relocation of the shaft log, and the v-drive, and possibly the engine. That's a bunch of work! Hope that helps.....