punch die question

Hi list. I need to MFG a bunch of AL discs without holes in them from
some .017 thick AL I have a 3 ton punch press. I need the discs 2-1/8"
dia and to be flat when done. My understanding is that the female bunch
needs to be bored out to 2-1/8 DIA but how do I calculate the DIA of the
male punch?
Thanks
Stan