Flue sizing

I have access to a wood fired soda kiln, based on Peg Udall’s design. We’ve fired several times and have trouble getting temp *10 on top. It was built with what appear to be very large flue openings. Is there any formula, rule of thumb for the size of flue opening?

Hi, unfortunately no one really uses this wiki anymore, it might be best to ask your question on Glazy: https://glazy.org Here is how to post a question: https://help.glazy.org/guide/posts/