Lay the casting down, hold a straightedge in one of the grooves made by the leade, take a good clear photo of it , blow the image up, take a piece of wire, bend it so it matches both the straight side and the straight edge. Google protractor or compass until you get an image with degrees on it you can see, hold the wire up to the screen and measure the angle, or print the compass out and lay the wire on the image until you can read the degrees.
When I needed a crowning tool, I used video of a 1911 firing in slow motion. When the boolit exited the muzzle, and the gas cloud followed it, I stopped the video and used a piece of wire, I bent the wire until both sides were parallel with the edges of the gas cloud, measured the degrees and divided it in half, came up with 22.5 degrees on each side, for a 45 degree included angle. I found a carbide NeWay valve seat cutter for a Yamaha engine was 20 degrees. I ordered this cutter, made a sleeve to fit over the Manson mandrels that index off the lands for concentricity, and I was in business.
I chose 20 degrees for the angle to use for the crown, if you look at that gas cloud, having the crown parallel to the exiting gas provides the least amount of deflection of the boolit from the pressures of the gas, there is no surface to introduce turbulence, there is no surface for the gases to push against that could possibly upset the boolit, and I can clean up most factory crowns with this without losing barrel length. It just made sense to me that both the angle of the crown and the angle of the gas cloud needed to be as close as possible to the same.
Now I am telling my trade secrets, but I included this info to illustrate how to use images and a simple piece of wire bent in the middle to transfer angles from a computer screen or photograph to a compass, so that they can be measured.