I Bought a used fork for my MTB and according to my LBS it is about 1cm to short and that he could put it together but he would not ride it. Is there a golden rule as to what the steerer tube should be below the stem?

I'm assuming it's a threadless steerer.
Common practice is to have the steerer - when desired amount of spacers are added - to end about 3-5 mm below the rim of the stem. Gotta leave a little space to ensure the ability to set the headset preload.
Never heard about anyone doing actual testing on how little overlap it'd be possible to get away with.
If it's a straight metal steerer, you can use a quill-type steerer tube extender to get some more overlap.

top of 2 stem bolts in the side has to be gripping the steerer, not above it.

what riding you plan? threaded you must have sufficient thread for the lock nut,

threadless, 1.125"?

you can order a BBB BHP 21 from some place like Wiggle in England and there is a stem raiser that can work.
its internal quill + shim sleeves for the stem to grip, the wedge bolt is internally threaded, so your headset pre-load is still thru the top cap, pushing down on the stem and the spacers beneath it.

threadless 1", there is a BBB BHP 20.








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