Rj : Congratulations on your 1st build ! You have not definitively stated that you have 'The Book', please checkout the site in the link below.
http://www.rocketstoves.com
Buy the Book,
Rocket Mass Heaters, read the book! This is the best starting place, and virtually guarantees that when you return to this Forum
you are using the same words to describe the same parts, and their sizes, shapes, and orientation to themselves, each other, and the Whole .
Understanding how the sizes affect the hole is not just an interesting intellectual exercise, while the math you need is some thing you learned in or about
the 5th Grade (and you are smarter than a 5th grader,

RIGHT!? ) Understanding where the ratios come from -Pipe diameter and length- is something
you should Know and be able to explain to your grandmother !
A final thought At Rocket Stoves dot com Leslie mentions You tube, in her own gentle way she is telling you that there is a lot of crap on U-Tube, and
anything you see there should be regarded with a great deal of suspicion !
Think like Fire! Flow like a Gas ! Don't be the Marshmallow ! For the good of the Crafts ! Big AL