RMH Dimensional Problem/Question Four or Five Fire Bricks across the Burn Tunnel?

Hello Good People,

I am in the middle to later stages of RMH construction.  I am attempting to construct an eight inch diameter RMH with a pea gravel mass bench made out of cement blocks.  I hope to have three of the liquid tote containers supported on top of the mass bench ( I do have a mason helping me out with aspect of the project)

My concern is that I am trying to replicate the VillageVideo RMH pdf plans for a 8" diameter RMH.  I have the typical yellow fire bricks.  When I try to get the inside dimensions shown in the attachments I have to use five bricks across the bridge instead of four which I see in the Ernie and Erica plans as well as in the Rocket Mass Heatersby Evans and Jackson.  It may be insignificant but I can for the life of me get it to look like the plans without the dimensions being off.

When I use four bricks across the top I can get the feed tube 7x7.5 like the diagram but the heat riser is 7.5x 9"
If I use five across the burn tunnel I can get 7x7.5 in the feed tube and in the heat riser.  Does it matter?  

I want to do this right the first time.  

Thank you for the help.

The missing second course will be added when mortaring this together, my bad, trying to save fire brick and not realizing that the tunnel would not be high enough to function properly.  

What are the exact dimensions of the firebricks you have? Is your burn tunnel 25" x 7 1/2" inside as the diagram shows?

There does seem to be an inconsistency in the diagram as standard firebricks are 4 1/2" x 9" x 2 1/2" thick, so four bricks would give a bridge 10" long, which requires two 7 1/2" x 7 1/2" openings to fit correctly. Five bricks would give a bridge 12 1/2" long, which would not fit at all well. I would suggest to use four bricks, as the shorter the burn tunnel the better, all other things being equal. If you want to stick with the feed and riser opening dimensions, you can make the overall burn tunnel 24" long instead of 25".

Than you for the dimensions. I knew I wasn't going crazy! My bricks are 2.25 inches wide instead of the 2.5 that is standard.

I was given the bricks so I just assumed that they were the standard size.  

Should be fine for heat and all that?

I really can't afford to purchase all new bricks so I will go with what I have.  Thanks for the info.