PerryBebbington:
A DC DC converter swaps current for voltage. If you feed 12V in and take 5V out at 1A then the input current at 12V will be a lot less. If there were no losses then the input current at 12V for 5V 1A out would be 5/12 * 1 = 0.42A (This is where some kind person tells me I got the maths wrong!)
No.. you got the math right !