Deadrise is typically measured at the transom. For a Mainship 350/390, that would be about 178 degrees. From the end of the keel all the way back (about 6 feet, the hull is almost flat.
I have used a digital angle measure for my table saw to measure it, but now you can use an app on a smartphone. The deadrise will most likely be in the 12 to 18 degree range.