ORIGINAL: TakeshiSkunk
One method I've seen is taping an allen key to the main shaft real snug and pressing it down against the plate, since its a 90 degree bend in the key.
That is the correct technique, but not because the key is at 90 degrees. The important part is that the swashplate is at the same distance at all locations from the key. The angle of the key is non-critical. Just spin the mainshaft/key combination while observing the distance between the key and the swashplate.
If I level it by say a small level....wouldn't it be "off" if the heli itself is not level or it's sitting on a surface that is not level?
Yes, you have to level the helicopter fisrt if you are using a bubble level. I typically place the bubble at the bottom of the main shaft just over the top bearing at the top of the frames.
Rafael