I think that it goes back to the old saying that you can make anything work if you have the time and patients. If you build using the calculators recommendation you can expect your pit to burn 2-3 small splits every 45-60 minutes at 225-250 degrees. If you have an oversized FB you'll have do some test burns to try and figure the required fuel and the timing on refueling.
Just to clarify the firebox should be about 1/3 cc then add 5 to 10%. So 35 to 37% of cc.
You're right, looking at the Texas 1845 the fb is 50% of the cc. I can't comment on who's right or who's wrong. Folks on this forum have been getting along really well with the rules of thumbs posted above. I'd say don't worry about those guys and go with the calculators.
FWIW I'm reverse engineering a competition smoker right now and as near as I can tell the fb comes in just barely under 1/3 the cc. (not sure what the ethics are on that..? The design isn't original anyway)
To Kcd2016’s point, you can definitely over think this. The calcs work. Has to be a thousand+ pits that have been built with these calcs. They are ideal for a first build for sure. There are a bunch of folks out there who say calcs don’t matter. You can guess and get close. I’m not smart enough to do that. Lol
I built a few different style smokers before I found out that there was a formula for designing them. I was eventually able to get each one of them to work but they all had some serious issues and I couldn't figure out why. I started searching the web and I found another site that had a calculator and that led to the discovery of this site. I'm not sure how many that I've built using the calculator but I haven't had any major issues with any of them so I will always recommend that a new builder sticks real close to the calculators sizing. Once you've built a few of them you start to figure out little tweaks here and there to make them a little better than the previous build. Anyone that has ever built a pit will tell you that there's a lot of work that goes into building one and the last thing that you'd want is for it to not work worth a darn when you're finished.
A lot of information on the internet...and yes, much of it is conflicting. In the end, you have to decide what you want to follow and what ideas make sense to you. However, pick one calculator or thought process and stick with it.