If you have a ski/snowboard straight edge tool (usually used to check base flatness) you can use it at the tip and tail to find where the Tip/Tail leaves the snow. this is the end of the contact point beginning of the true running edge. Run the straight edge along the edge where it is in contact with the base, you'll easily see the edge curl away from the straight edge.
I have a Rossi WC 184 and a 190 (as too Pat, above) I rode them for a very short time going by the hole pattern. That was a big mistake. They rode well but they didn't feel great. I then measured the running surface and found the true center, which by the way matched the true center of the side cut also. I mounted my stacne to match this real center and I am on both boards at the most forward set of holes possible now. There is a world of difference now and I believe this is where they should be ridden. Even the 190 is so smooth I love that thing now. Transitions are way better than they were in the old mounting position.
Also as quick reference I have F2's 186 and 172 and their hole pattern lined up just about perfect with the center of the true running length. I don't know why the Rossi's were so far off, the 190 was twice as far off as the 184, but they sure ride good now. I have a funny feeling that Rossi just mounted their jig to the tail and they used the same jig on all their boards so as the boards got longer the stance went farther back...bad bad!!!! (Atleast for me that was bad)
Joel