You don't need to trim the leader, and don't need to use a card to load a Barnack Leica. I just take the lens off and set the shutter to 'T', load the film and check that it has engaged the sprockets by looking through the shutter. While sticking a finger in to 'adjust' the film should be done with care you can only trip the shutter curtain if you actually turn the slow speed dial away from 'T', so its not likely. The big advantage is that it is a 100% certain way to get it right and avoid sprocket holes appearing in the pictures.