Be VERY careful when you choose the Greenlee knockout punch.
There are 2 different punch sets available.
The most common is the Conduit Punch set and this is the one to be very aware of and very careful when using it.
The Conduit Punch makes a hole that is the "Nominal Size" for a conduit fitting.
A 1/2" conduit punch makes a 7/8" hole.
A 3/4" conduit punch makes a 1-3/32" hole.
A 1" conduit punch makes a 1-11/32" hole.
A 1-1/4" conduit punch makes a 1-11/16" hole.
I strongly recommend that you measure the die before you attempt to make the hole. Once you make the hole and it's the wrong size? Yea, good luck on that one.