The 42 circuit limitation applies only to 'appliance' panels .... that is, panels whose circuits have neutral wires. That's what the 10% rule defines. Other panels are "power" panels.
As mentioned above, this arbitrary rule is about to leave us.
Also as mentioned above, 'legal' and 'good design' are not always the same thing. Personally, I will even go so far as to assert that a design that is deliberately intended to 'push the code' envelope is a bad design for every other purpose.
