This is a very important question, especially if you're converting a firearm to a different caliber than it was designed for.
I always ran under the assumption that the rearward force was figured by the square inches of area times the pressure of the inside diameter at the base. Because that's hard to do I always just measure the base. I have never use the rim in any calculations.
I'm not sure if I'm right or not and just don't have time to look it up in my books but if anyone is going to convert a firearm I would certainly get some good information before I did it.
