I do, it's out of one of my own aeroplanes as well.
Damned silly, some previous owner / maintainer of the beast decided to use a quite ludicrous amount of permanent threadlock on the calibration screw - works fine apart from being permanently 13mb out of calibration. I couldn't think of any way of removing the threadlock that wouldn't probably nadger it even more (sealed unit, screw is surrounded by the plastic sealed casing), so relegated it to paperweight. A serviceable second hand replacement only cost me £25 so it wasn't that bad.
G
N.B. This reminds me of the time I filed an airprox after some pillock tried to fly into me off the Scottish coast. I had a worried investigator phone me up..
"Sir, you say in your report that you were at 900ft QNH"
"That's correct"
"But you don't give an altimeter setting"
"I don't recall what it was"
"Where did you get it from?"
"It read zero halfway between the low-tide and high-tide marks just before I took off from the beach"
[Long pause
"Err, there may or may-not be something wrong with that, I'll get back to you".