I think the information you want can be found in Peterbergs post on the first page of this link.
http://donkey32.proboards.com/thread/337/helpful-hint He has a formula and an example. Here is his post that I have cut ,pasted and added quote marks.
"Very true, Donkey. I've attended a rocket mass heater workshop in the Netherlands last week and we've had a heated discussion about this very same point. I'd cut out the opening at a little bit larger than system size, and the instructor of the workshop insisted it was far too small.
This is what I've learned this week:
There's a difference between a stream opening and a stream profile. The main culprit is the gap between insulation canister and barrel. When you're only looking at the opening in the side of the barrel, it's easy to think you have to make the opening the same as system size. This is utterly wrong, because the gases are not streaming straight out of the opening. Instead, it is coming from left and right, and from the top. Moreover, in the corners two streams can't pass there at the same time, so you have to compensate for that.
Maths can help here. Start with system size area, divide it by the gap between inner and outer barrel, add twice the size of the gap for the top corners and you've got the length of the stream profile. The profile length consists of the top rim and sides of the opening.
For example: system size of 8" equals a little bit more than 50" square. The gap is, say, 2" wide, which will get us at 25", adding the gap twice will give us 29". Presuming the opening is one foot wide, the height need to be half of 17", which is 8.5 inches. The open area in this instance will be about 90" square.
The resulting opening in the side of the barrel will look enormous, nevertheless this is absolutely the correct method. The recommended gap is smaller than in my example, so the profile length will be even larger.
P.S.
The top rim is rounded, so you have to measure it along the contour, not as a straight line."
I had to read it through a few times to get it. Using his example helped me sort it out.
Good luck and welcome abord.