Actually - to calculate maximum theoretical THRUST on the bolt face or standing breech, you must use the internal sectional area at the base of the cartidge.

In practice, this is mitigated by the strength of the cartridge case, frictional forces between the case and chamber wall, and degree of case taper (heavily tapered cases transmitting more "bolt" thrust).

IMO - What allows some of these dodgy conversions to work, is the ability of the case to contain backward thrust!

Some gunsmiths employ this principle to do a rudimentary "proof test" by oiling a case and firing it, on the assumption that if the maximum theoretical thrust is allowed to develop, this approximates the thrust generated by a proof load under normal circumstances.

So, if you are shooting one of these marginal conversions, you had better make sure you clean the sizing lube off the case, and clean the oil out of the chamber before you pull the trigger! And - oh! - don't polish your cases, or use Nickel plated ones!

BTW - the current CIP Minimum Proof Pressures (PE) for Centrefire Rifles are 30% over the Highest Mean Service Pressure (Pmax), and only 15% over the Highest Individual Pressure (PK) - not much of a margin!