What height should I set the cavitation plate on this boat?

Old 3 point, it has three flat planing surfaces the front two are about 4" lower than the main hull. Should the cavitation plate be mounted at the transom height or the height of the front planing surfaces?

 

It will be all trial and error to get it dialed in, and will depend on the exact motor, load, prop, speed, where you sit, how you plan to use the boat etc.

 

It's a ventilation plate and it should be approximately an 1" above the bottom of the transom bottom, on the centerline, if the keel line was extended directly aft. The actual placement will depend on how she performs, so typically a jack plate is used to find the sweet spot, which could be about anywhere across a few inches.