It is hard, because manufacturers don't publish the expected TPD, they publish the maximum the device will draw. Actual usage depends on a lot on how the computer is actually used.
I also agree the online calculators suck. They either assume you are going to be using a crappy power supply or they are trying to sell you a bigger supply than you need.
That is without getting into the distribution of available power. Fans, Video cards, and modern CPUs (since the P4) draw most of their power on the 12V rail. Support chips will likely be using 3.3V or 5V. Modern memory uses something like 1.8V: whether that comes from the 3.3V line or 12V line is probably up to the board designer (using 12V would substantially reduce the required current, but at about 15 Watts, that would only be about 4.5 Amps at 3.3V).