What is the proper central bank seigniorage?

About a third of the typical ATM fee for paper bearer cash.

It is a simple process, we are not the dominant apple pie maker. ATM fees have a cash handling fee, a bank fee and a monopoly fee; there is not much more to it. Split them in three, as the monopolists would. I get a half point a year. I am in the ball park, and the Fed generally collected that during the last crash.

Sandbox free currency bankers drop that way down, to almost nil with Secure ID. Sandbox is the theoretical model, self computed in run time, for real.