"The pin numbers I'm talking about are the "imaginary" ones I need to put into the code of an arduino sketch to address the correct "real" (electrical) connection on the board. I'm surprised that I can't seem to find a cross-reference for this anywhere."
The concept of using 'virtual pin numbers' Vs physical package pin numbers is so that the software functions can stay consistent and internal software mapping functions can resolve the hardware differences between many otherwise software compatible AVR microprocessors.
Otherwise one would have to modify or load a completely different version of the Arduino IDE for each different board if it used a different mega chip.
Some people use the term 'leg number' when they are referring to the physical chip's pin numbers and 'pin numbers' when referring to the software virtual pin number. It may be confusing when first learning but in the long run it makes things a lot more easy to use and less to memorize when working with different versions of the AVR mega series. Most designers of Arduino boards label the 'virtual pin names' near the connector pins, so that one doesn't really need to know what physical pin number (leg number') that it routes to. One really only has to deal with this if there are building there own board from scratch.
Lefty