Make sure you get a large metal file, where with one stroke along the flat edge a visible reduction to the base can be achieved. Sand paper is a waste of time.
Personally, I first bevel the base all along the bottom such that it 1) gives me a clear mark of how much metal I want to remove overall and 2) makes it far easier and quicker to remove the rest of the bottom and still leave it flat (because there is less surface after it has been beveled).
You can remove easily 20% of the bottom of the base by beveling first because the bottom of the base is a right angle and when this is dragged along the file, it comes off in a flash.
If you know what I mean...