The rule of thumb is you can mix any two that are next to each other on this list:-
The reasoning is that the further they're separated, the harder they are to mix. Perhaps if you spent the entire afternoon with your stirrer/whisk you might get there.
The same rule of thumb says the more matt finish will dominate. ...
Mixing paints of different sheen to get a specific sheen can be done but it will be hard to match in the future.
The desired sheen is obtained by amount of binder and pigment in the paint, and mixing paints with different sheen does allow you to shift the sheen without compromising the chemical properties of the paint.
When repaired patches are later re-...