I'd be interested in learning a lot more about the inner workings of the Perfectflite altimeters, as my main project involves sending telemetry from them.
Rocket altimeters generally spend some time on the pad reading pressure, averaging them, and then deciding that the pressure they're reading on the pad will be an altitude of zero (AGL, of course). I use Perfectflites and Missileworks, and they both are very good at what they do (though of course I have no idea how accurate the altitude readings I get are).