It needs to make a good deep connection on the head where it slides into the head. If it sticks out too far at the top then the little grub screws will not grab the collar ring and hold it and the whole thing will be too high. You will be able to move it up and down and if it runs long on the bottom a little it will not matter. I suppose a good way would be let it stick out at the top and keep the bottom lose, then just push everything down, and it should self-adjust, then tighten the bottom nut. There are not difficult to wiggle up and down and you will find the point you need without much trouble. Just let stuff float around some until your happy. When you do the last setting on the grub screws keep the wheel straight and tighten around the wheel. The last time I did it, I move the wheel to make it easy on me, but it pulled the lower piece that shuts the mechanical part up. On the final setting that piece needs to point down.
Steve