Calibrating tachometers?

Steve, 4spdragtop ask me about calibrating a tach

I'm trying to come up with ideas that the average guy could employ to do so.

There's always "find a guy" with a good tach. How do you know? (It's a good tach)

Or you could "find a guy" with a tuneup machine, but that cost's money, most of the time.

My Fluke won't trigger off the ignition, and most guys don't have access to an o'scope or signal generator

What I'm thinking is

Most of us have laptops or at least computers with soundcards

There are numerous places to download various sound tones on the www

A Radio Shack audio transformer to step up the audio voltage, maybe a diode, and hook to the tach trigger wire

The old Knight Kit/ Heathkit tachs calibrated right off line voltage with this:

The problem is, 60 hz AC generates a very low RPM, 900? I believe for a V8

(60HZ X 60 seconds in a minute / 4 because a V8 fires 4x per crank revolution)