Perhaps I'm not sure then!
Specs for the Arduino indicate a recommended input voltage range of 7-12V; surely the best I could do is a (presumably rare) 7V regulator, then throwing away 2V with the Arduino's regulator?
I understand the Mega 2560 runs on 5V; what's the best configuration of components to step 24V down to this?
Ideally I don't want to replace the regulators on the Arduino boards themselves due to the number involved in my project.