Flintfire you are right, powder manufactures curing specifications stipulate the temperature of an object's surface, in our case the bullets surface.
The thermocouple cast into a bullet is just a thermometer that is reading the temperature inside the bullet, but once the center of the bullet thermometer reaches 400 you know the bullet surface has already reached that temp. Since all ovens, even convection, have hot & cold spots placing the bullet thermometer in the coldest spot of the oven pretty well assures when it reaches 400 all the other bullets in the oven have also reached 400. So timing from that point assures we have met the manufacturer's specifications. If still in doubt add more time.
The whole point to my mentioning making your own bullet thermometer is it is an easy way to monitor the curing process regardless of any changes made in the future.