Dave:
If you don't cut the housing all the way across, and just do a 6" housing then there is a possibility that the 6x6 could be bigger than 6". And then it won't fit.

You'll know when you do a frame fit up, and you can then adjust as needed.

But if the door post is less then 6" and you cut a 6" housing then you'll have a gap. I know it's just a shed.....

One solution is to do the reduction to 5.5 on two faces, then cut the housing to 5.5" on both sides and you'll know it's the right size. The first reduction is on the face opposite the reference face and the second reduction is on the face opposite the adjacent face.



To insure that the door would fit between the posts, if you're doing that type of door, I usually layout the adjacent face of the two door posts towards the middle of the door.

In the above drawing the post shown would be the left side post.

Hope that helps...

Jim Rogers