In general you can take off 6mm of each side of the door maximum, you should always take equal amounts off each side of the door (if in doubt check, call or e-mail for advice).
When you need to reduce the height of the door it is best to consider that the same rule applies if your door is a veneered,
as you do not want to cut past the veneer or make it too thin.
If it is a solid wood door with no veneer then you have a better option but never cut to close to the joints as this will weaken the doors construction and could lead to the joints coming apart.
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