Dimensions for 6" core? Template for form?

Now that we decided to make our own core using fireclay, perlite and refractory cement (as discussed at https://permies.com/t/41987/rocket-stoves/Substitute-adobe-bricks-fire-bricks), I'm still confused about the dimensions.

Matt Walker has a great video at https://www.youtube.com/watch?v=7ANMXGrxgnE including the dimension for the 8" core.

I've heard/read numerous times that the 6" has to be "just right" and since we won't be able to reuse the materials if it doesn't work, I'd like to do it right the first time.

We have Erica's 6" Annex plans, but the drawings with the bricks are clearly not to scale and lack the height. Any help is greatly appreciated!

Christine, here's a not very good drawing I made to scale my design down down to 6". Dimensions are for the inside spaces of the finished core.

Thanks, Matt!

I just posted some drawings at http://highdesertpermaculture.org/blog/2014/12/09/dimensions-for-a-6-rocket-mass-heater-core/ and greatly appreciate comments from you and anyone else with feedback!