Really no need for a key, but unless you have a degree wheel, it's a guess. I made a pointer and Mark on my crank pulley so I can easily find tdc, then marked off (by 2° increments up to 32°btdc on the pulley). Have a Mark on the flywheel that lines up with a certain spot on the coil. Pretty easy to adjust once the homework is done. Of course this only works with this one engine/pulley combo. All that once the flywheel is lapped to the crank.