in the world of shearing steel we use the "6 to 7" rule, which tells us that a good place to start is 6 to 7% of the material thickness. when shearing harder alloys (in this case A50, as opposed to A36 steel plate and angle iron) the appropriate starting point is closer to 10%.
now : a punch is really a shear, just formed into a circle, so the 6% needs doubled to give approx die clearance.
in the final decision, it is entirely appropriate that die clearance be determined not by formula in any particular application, but by experience and experimentation. there are excessive variables to allow a simple formula to work optimally in All situations.
still , 6 to 7 yields a fine starting point.
Harbor Freight = garbage. top to bottom. if you liked fish, you would find most of their inventory is not fit for reef construction.
HF is a blight on the economy of the planet, and present an excessive challenge to landfill reform.
if peter was president. Harbor Freight in it's corporate entirety would be shoved onto a very old ship, and sent back where it came. if that country refused shipment .. reef time. (pity the fish..)
HF ? .. not a problem.
our own Ken T. once again shows us the way, and this time it's how to drill thin metal.
Ken is correct. a twist drill is not what you want. in fact, a twist drill is exactly the tool most NOT to be employed in thin material drilling. although a noble item, the twist drill is simply not the correct tool in this instance.
what you want is the Step Drill, commonly know as a UniBit.
the U-bit can still rumple up shim thickness materials though.
ohhh.. that to do ???
i'll tell ya what to do. go to Lowes (less bad than HF) and buy a sheet of 1/4" .. they will call it masonite, but it's really a composite of wood fibers that look like masonite.
a 2 x 4' sheet is like what ... 6 bucks. it's nothing, and you can pick it up on the way home from work. the stuff cuts like a dream, drills just as nicely, and has enough balls to clamp and hold your tin properly.
sometimes they call it MDF. but real traditional MDF is a thicker and quite Vile material used by cut-rate mexican roofing contractors to do shlock work.
the material you want will be found at Lowes and Home Depot (currently recovering from criminally bad corporate management) to have smooth surfaces on both sides,and already cut to the easy-to-put-in-your-car 2 x4' sheets.