There are two methods I use to do the fins for my sims. The manual method, and the digital.
Manual:
I take an image of the fin with a ruler (preferably along the root edge), scale it/print it so that the image's ruler matches mine. Next, using a ruler and a protractor I draw a line along the root edge, and connect each point of the fin to it with a line at a right angle. Then I measure the height and and distance of each relative to the point where the leading edge meets the root edge.
Digital:
Take an image of the sim with a ruler, and crop out all of the "not fin" by making it white, and change the fin's color to black (I do clean up the image as best I can), making sure the root edge is even with the bottom of the image. I import the resulting image in OR, then scale the results to match a known dimension (usually the root edge, but sometimes the height (when the root edge isn't straight)), and eliminate any odd artifacts (points) that are created by the process.
My DRM fins were based on a digital image. I still have the black and white image I used for it.
I had to convert this from the .bmp image I used to a format the forum can display. It was derived from an image of the DBRM fins I found on one of the online archives. The DBRM uses the same fin, and is basically the same kit, but with an 18" body tube.