As long as your scene units (Properties > Scene Properties > Unites) are set to Metric, there should be a direct relationship between the value you want and Thickness. The other main option that has a direct effect on the result is Offset Even. When on, your walls would have a thickness of 1mm; when off, the corner edge would have a length of 1mm instead:

Be aware that Scale of the object does not affect the values shown here until it's applied. A plane of 2m with a scale of 2.000 would still show these same numbers in Edit mode, but they would double when the scale is applied.

As long as your scene units (Properties > Scene Properties > Units) are set to Metric, there should be a direct relationship between the value you want in mm and Thickness. The other main option that has a direct effect on the result is Offset Even. When on, your walls would have a thickness of 1mm; when off, the corner edge would have a length of 1mm instead:

Be aware that Scale of the object does not affect the values shown here until it's applied. A plane of 2m with a scale of 2 would still show these same numbers in Edit mode, but they would double when the scale is applied.