The nail in the wall thing is a total myth and doesn't work? I can't believe that people by into a myth like that. There is No way possible to balance something with out finding its center, that's a proven fact and in order to balance a mower blade you have to find the center of rotation.
Uh... That's kind of the idea, isn't it.
There's no myth. There is a chance that you won't find the center, of course; if your blade has a toothed center hole the thing will snag. And a nail provides a very small balancing point which is a bit off center. But if you can create a fulcrum, you certainly can do static balancing because of this thing called gravity.
In essence, you choose the center of rotation as the same place the mower shaft will establish it the instant the blade is mounted: in the exact in the center of the mounting hole, or as close as you can get to it. Once you have that spot established, it becomes the reference from which you balance the blade.
A more accurate way to do this would be to slide the blade on a horizontal shaft or rod approximately the same diameter as the mounting hole, perhaps held in a vise. That's instead of the nail. It could be a long socket, for instance. Find one that slides easily through the mounting hole in the blade, hold it horizontally in the vise and spray a little lube on it. Slide the blade over the socket and let go. If the blade is heavy at one end, that end will move towards the ground and you need to remove a bit of material until both ends appear evenly balanced. That's static balancing.
The nail is quick and dirty but does nearly the same thing. So will balancing the blade on a bit of pipe sitting on the top of the bench, if you can get it to stay still long enough. The point here is to remember that the only good tool is the one you actually use.