I agree 12 seconds isn't bad. It might be reset to a closer error at the watchmakers but almost always will be different on your wrist.

I like to play around (it's what amateurs do) with adjusting my watches to beat more closely to the ideal. I used to wait hours to see if the continuous second hand was + or - relative to the reference since electronic timers were way too expensive at thousands of dollars to be considered.

Then somehow I discovered the affordable Microset timer (link) for $295 which has opened up a new field for playing around, but still with the same on and off wrist problems. Pic. 1

Today I had a new aquisition (not the Carrera shown) that was running + 19 seconds per day. In sequence I adjusted it to -12, +9 and finally settled on -5 seconds per day as good enough. This was at room temp. After wearing it for a few hours I did a spot check to see the effect of body temp and got +1 second per day error.

In contrast I have a desktop set with the watch adjusted close to +- 1 second which is spot on week after week. Pic. 2

My recommendation is to learn to accept these inconsistencies of mechanical watches as it will either drive you nuts or back to electrics which might be worse.

David
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Pic. 1

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Pic. 2