I used this trick for spacers as short as 1/8" and it worked fine. You would think that the brass tube would "drill itself" down, make the hole deeper and thus make longer spacers. In actuality, the blade prevents this from happening. The jig could be improved using real wood instead of MDF --a nice maple or ash would be great, but for now, it seems to work fine. The blade used in the video had already made around 40 cuts to the copper tube. --Pretty dull, actually and it still works fine.
https://www.youtube.com/watch?v=pxuZf3gCOkU