Gooday.
Some times you have to do them by "eye" so that they look level.
If you can't eye them in try this: measure the height of each door and place a mark in the corner at both heights. Use a string line from each door and split the difference of the measurement.
BTW you are lucky it is only 10mm, I have seen some where it is 50mm and that is new houses.
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Ian (Robbo) Robertson
"We do good turns every day"