Yeah, the Lanksy isn't a very exact system in terms of the indicated angle actually being the sharpening angle. There are a lot of variables. But it's ok, because Spyderco's are sharpened by hand at the factory and the bevel angle isn't the same from blade to blade. It might be anywhere from 14 to 22 degrees per side, depending on how it was sharpened. The angle might vary from hilt to tip as well. This is pretty common.
Luckily, with decent blade steel (which the PM2 has), it can easily take a 15 degree per side edge and still remain durable for most cutting tasks that aren't crazy. So use the sharpie trick to see where you are. Get close to the factory angle if you'd like. It's a good place to start. You can move the angle down from there as you decide to experiment and see how low you might want to make the angle.
As Murray Carter said: I've almost never seen any one make an edge angle too low. If you ever do, I'll say you did a good job. Then you can just raise the angle slightly, grind in a microbevel, and be right back to a stronger blade shape.
Brian.