Central air intake distance

[clouvis]

Hi! This is my first post and I hope someone will have an answer to my question.
I am confused about the distance between the center intake tube and the baffle, or the cover. My collection system works with 4'' tubing. The problem is that I seem to get too much of the fine dust into the collection bag - about 10%... The central tube protrude 1 1/2'' from inside the cover. Should it be longer or shorter?
Thanks in advance.