By the way, if anyone is trying to design a number pad or calculator keyboard or similar for personal use with the goal being comfort/speed after retraining rather than prior familiarity, I’ve found it works much better to put two numbers on each finger.

Something along the lines of http://i.imgur.com/ofeQEC9.png

This is much better (faster, more comfortable, fewer errors) in my experience than the number row at the top of a keyboard, or a standard 3x3 + 1 grid calculator, computer, or phone keypad.