a general rule of thumb is the Prop to Pad should be around 3 1/2" below pad. Here is a diagram how to measure.
Take this measurement while the trailer is on a hard surface. Level the boat and then trim the motor so it is vertical to level or you might say straight down.
As I recall you do have a jack plate so it won't be too tough to move up or down depending upon what your needs may be. Things to observe after setting your starting distance is to keep an eye on your water pressure. If you get the motor too high you will see a drop in water pressure immediately. If you are too high you are also likely to have a rooster tail above the height of the motor. These are just key indicators that let you know you may be too high. If you are too deep then you may notice your boat will tend to plow a a bit and not stay balanced on the pad especially at slower speeds. After all this and you feel the setting are in a good starting point you other indicator will be the RPMs at mid range and wide open throttle. All of this things will play into your adjustments so you just have to be a bit patient to achieve the ultimate sweet spot for the performance you are expecting.
Once you get the settings finalized then you can calculate more accurately the overall performance of your prop based on optimum performance. Not really as complicated as it sounds.
Bottom line is the correct set up is critical before you can really judge your total performance. Sometimes trying to achieve a couple more miles per hour just won't be worth it if it takes you out of your comfort zone.