For more than a few rivets I make a small split sleeve in the shape of a tophat. That is used to hold the rivet in a collet on the repetition lathe and the rivet can then be faced/parted to length.
There are handy calculators for the length of rivet required, but it depends upon the exact snaps used, commercial or homemade, and is reasonably critical (within 5 or so thou) if a properly formed head is required.
Andrew