If I am remembering my old sheet metal class correctly, it was to hold
the amount to be peened just slightly over the diameter of the rivet
for a round rivet, made by peening to rough shape then finishing with a
rivet tool which made the domed head shape nice and round.
For flat rivets like we use in sheet metal work today we use almost the
same about one diameter past the metal to be held.
Assuming you are making this rivet yourself out of some rounded stock,
make the head for one side of the rivet in a vise so you can get it
exactly the way you want it. Then cut it for length and apply.
It is too difficult to take a small piece of rod and put it where it
needs to go and peen both sides well while on the project. You should
try forming a head to test your rivet metal. Depending on the metal,
the shape of the head and the profile you are after, the metal can
crack at the edges or you may have to hammer it hard enough that you
will distort the shaft making good alignment impossible.
The shop made rivets we used to buy were a very, very mild steel with a
flat side (for the vise anvil or a real anvil) to sit on while forming
the head. We only used home grown rivets when we ran out of these, or
made brass ones out of welding rod when we ran out.