Extra space in my feed tunnel

Hiya,

Dry-stacking my firebox to see if things will work, I find that I'm getting an extra quarter to half inch in one dimension of the feed tube.  Here's an image which probably explains more clearly than words.

https://imgur.com/KtNnmhs

Is this going to go away when I build it with the fireclay slip?  Probably not since that's meant to be very thin, right?  I noticed something in the builder's guide about protecting the first bridge brick by setting the brick above it forward slightly.  Would a quarter inch be more than "slightly"?  Is there another solution here?  I also noticed the builder's guide suggests that dimensions varying by up to half an inch don't dramatically effect performance/operation... so maybe it's close enough?

Hi Jean Paul;   I would say that you are close enough.  There will be some change with adding clay but not enough to worry about.
As far as protecting the first bridge brick. Rather than spacing the next brick, make them even and build yourself a P channel from 3/16" plate steel.  Check out this post   https://permies.com/t/100852/Peter-channel-dragon

This is the recommended way to get fresh air and create turbulence in the fire.  Also carving a "trip wire" into the roof of the burn tunnel help create a better burn. You will find info about this in the same post.