Mike,
It is not that there is a difference in conversion of pressure units. The problem is a difference in the method of measuring. tpi means that the pressure has been measured with a copper ring at the base of the cartridge. Then cartridges have been tested with a copper piston at the side of the shell. When you measure the same cartridge this way, it does not show the same pressure. The difference is not linear or a fix data, but depends on a lot of influencies. It is around 17%. (plus/minus.. to give you an idea)

Nowadays, at least in Europe and other CIP countries, the cartridges are tested at the side with a piezo element. The piezo element shows higher peak pressure at the same cartridge as it works “mass free”. Therefor some pressure data given for a cartridge have changed. This change is only related to the method of measuring and does not change the real pressure inside the cartridge.
So here you find our problem. You can not be 100% sure that a load developed today is within the pressure limites for an old gun with data given in tpi. You just cannot measure it the same way. (or do it the cheap ;-) way as Graeme did)

Hope that helps
Tom